I have not experienced this problem with BCC Title Studio in either VP14 or 15 at 1920x1080/25fps, but you might want to check the following to see if any of these settings are the cause:
If Title Studio has been added as an FX to a Vegas Solid Color (e.g. black) from Media Generators, is the Frame Size of the Solid Color set at the project size of 1920 x 1080?
In Title Studio itself, are the Width/Height/FPF/PAR settings under Edit/Project Settings at 1920/1080/your FPS/Pixel Aspect Ratio (PAR) as per VP14 Project Properties?
Again in Title Studio but under Edit/Preferences/Tools, is the Aspect setting the appropriate setting? Mine is set at the factory default of Square Pixels, but I haven't experimented with the other options.
I had a problem with a rotating sphere in title studio appearing a different size in the Vegas Pro 15 preview window compared to the Title Studio UI. The text was just fine though. It turned out to be a problem with my Vegas Pro preview window scaling to fit the preview in the window. If I set it to Full rather than Auto, everything matched up fine. I use Title Studio for all my projects at 1080p and it has been very reliable.