I'm new to RR, I'm trying to render multiple Gopro files from a track day motorcycle session.

I managed to group the video files and sync the GPS data for track map overlay.
Everything looks as I want it.

The problem I have is with final result of the rendering.

Original files are filmed with same settings during a 30 minutes race.
So original files details are: GoPro 5 black, 1080p, 90fps, bitrate is 60.056kbps

If I choose in render screen 1080p x 60 fps internet mp4, I see that bitrate gets set to 23000kbps. But the result is bad, the video is grainy and jagged.
If I manually set 60000 kpbs bitrate in rendering and I do a sample lap, everything looks great. It creates a 800mb file for 2 minutes video, but with expected quality.

All is good, and with the same settings I try to render the entire session (26 minutes), everything goes OK no errors during the rendering.
It finishes, but the result is bad.. Not as previous sample lap.
When I check video description I see bitrate is decreased to 23000kbps. And the quality grainy and jagged during fast corners.
Also, one thing I noticed, the resulted file is 4Gb size.

I tried many other rendering combination and any setting that looked good in the sample file, when rendering it all, result is bad and bitrate decreased.
File size never goes over 4GB.

Seems to me that any files that would result in bigger than 4gb get the bitrate automatically decreased... Can this be true?

I can't find anything on this issue.

I have ultimate license of RR3.
Win7 x64, 32gb ram, ssd, etc

Any insight would be apreaciated.