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I believe you rendered an Uncompressed AVI file. It would be at least that large. The file sizes are the yellow bars in the chart:

But without seeing those FILE PROPERTIES, we will never know....

There are many, many codecs that can go in an AVI wrapper. Some are compressed, some are not. Uncompressed files have a higher bitrate than compressed, so you can see from the equation that the file size will be larger.

https://www.vegascreativesoftware.info/us/forum/speaking-good-video-a-beginner-s-guide--104463/

Again, welcome to the discussion.

Thanks, and I should know better than to forget to post the properties, sorry!

Actually, you should post your RENDER settings rather than the project properties. Screen shot is best.

Remind me: what exactly is your question (briefly)?

The Mediainfo properties are the most important to see in this case. Thanks.

The Mediainfo properties are the most important to see in this case. Thanks.

Please accept my apologies for my late response, the man-flu hit me hard!
Here are the Mediainfo properties as asked for.
Thanks for the chart, but, I'm embarrassed to say, I don't really know what I'm looking at!

I'm basically looking for a good intermediate to use for further editing and am shocked that this file is so much larger than the original footage.
As I already said, maybe there's something basic I am missing, but surely any original footage should be the largest file (obviously if rendered with no effects etc)
I simply want to put the footage in the timeline, cut out the stuff I don't need, render it to a lossless (or near lossless) format and edit it further with other footage.
Thanks as always for any help and input.
 

General
Complete name                            : C:\Users\Chris Hiles\Videos\New folder\20190119_173407.mp4
Format                                   : MPEG-4
Format profile                           : Base Media / Version 2
Codec ID                                 : mp42 (isom/mp42)
File size                                : 48.2 MiB
Duration                                 : 14 s 357 ms
Overall bit rate                         : 28.2 Mb/s
Encoded date                             : UTC 2019-01-19 16:34:22
Tagged date                              : UTC 2019-01-19 16:34:22
xyz                                      : +47.9413+010.5339/
com.android.version                      : 8.0.0

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.2
Format settings                          : CABAC / 1 Ref Frames
Format settings, CABAC                   : Yes
Format settings, RefFrames               : 1 frame
Format settings, GOP                     : M=1, N=60
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 14 s 292 ms
Bit rate                                 : 28.0 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 60.000 FPS
Minimum frame rate                       : 58.594 FPS
Maximum frame rate                       : 61.560 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.225
Stream size                              : 47.8 MiB (99%)
Title                                    : VideoHandle
Language                                 : English
Encoded date                             : UTC 2019-01-19 16:34:22
Tagged date                              : UTC 2019-01-19 16:34:22
mdhd_Duration                            : 14292
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 14 s 357 ms
Source duration                          : 14 s 363 ms
Source_Duration_FirstFrame               : 6 ms
Bit rate mode                            : Constant
Bit rate                                 : 256 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 449 KiB (1%)
Source stream size                       : 449 KiB (1%)
Title                                    : SoundHandle
Language                                 : English
Encoded date                             : UTC 2019-01-19 16:34:22
Tagged date                              : UTC 2019-01-19 16:34:22
mdhd_Duration                            : 14357

"maybe there's something basic I am missing, but surely any original footage should be the largest file (obviously if rendered with no effects etc)"

You may be missing the fact that file size = bit rate x time. The typical camera media like yours is highly compressed compared to the number of bits it takes to describe each pixel in the video if it was captured uncompressed or decompressed after the fact.

"I simply want to put the footage in the timeline, cut out the stuff I don't need, render it to a lossless (or near lossless) format and edit it further with other footage."

Therein lies the problem, lossless or near-lossless will always be larger than your original media if no cuts are made and the time is held constant. Intermediates will have the benefit of playing well on the timeline, but small file size won't be the major draw for any intermediate.

If you're able to edit your camera video easily and just want to trim the fat from you clips, you might consider a program that "cuts on an I-frame" without re-compressing the video. I use a program from my camera manufacturer that does just that and I use a feature from Happy Otter Scripts called SmartVideo Trim to trim the fat while not re-compressing the original video.

I'm basically looking for a good intermediate to use for further editing and am shocked that this file is so much larger than the original footage.
As I already said, maybe there's something basic I am missing, but surely any original footage should be the largest file (obviously if rendered with no effects etc)

Compression makes a file smaller. Your mp4 file has been compressed to 1/35 of its raw uncompressed size. That is because you used a compressor called AVC to make it smaller Just like we all do.

An Intermediate file is not as compressed as your mp4. That's so it doesn't lose so much quality. Maybe only 10:1 compression.

Therefore, the less compressed file is much larger than the more compressed file.

If you had not used a compressor for your mp4, it would have been 17 GB! So if you want a larger original, render uncompressed.

Thanks for the chart, but, I'm embarrassed to say, I don't really know what I'm looking at!

Chris, look at the yellow bars on the chart. They tell you which Intermediates atre larger, and which ones are smaller. If you want the smallest lossy compressor I could stand, choose 422 for your intermediate. For your concerns, that is all you will need.

Also, your file looks to be "variable frame rate." You will probably need to fix that going in. Lazy way is to run it through Handbrake. Search for separate discussions, it's not covered in this topic.

The "not so easy" way to convert your video to a constant frame rate and all I frames might start with an FFMPEG script like this. Copy the contents below into a Notepad file, edit to suit your files and save as Not So Easy.bat. 

ffmpeg -i "Your input file name".mp4 -c:v libx264 -profile:v high -level:v 4.2 -crf 18 -r 59.94 -acodec copy "Your output filename".mp4

Pause

The Pause leaves the window open so you can see all the error messages you're gonna get.

Wow, you guys are amazing!
Thanks for all the great advice and help.
The HOS look really interesting, I'm looking forward to trying those.
I understand the chart now, thanks.
So, "constant frame rate" means faster/better/easier editing/rendering in Vegas?
I've used ffmpeg before to inject 360 video info into an edited file, but I don't know what you mean about the Pause and seeing all the error messages.
I still have a way to go!