It seems to only do this with the HEVC codec but I specify for it to use 28 Mbps CBR, then in the properties (after video's rendered) it says 120 Mbps and the file size is 4.84GB for a video where the source was 2.08GB.
This is frustrating- I'm trying to transcode to HEVC specifically because it's able to have a lower bit rate while having high quality.
The source file resolution was 1440x900, there is a jump to 1600x900 but that'd be a very small change. Source file was also 60 fps.
Otherwise, here are my settings.