Well, I don't know what recording software you use, but I use fraps so I get that too. That's because fraps records in a uncompressed format which needs to be coded and compressed by any video editing software like Sony Vegas, Adobe Premier Pro, etc.
Hope this helps!

Then you might need to revise your rendering settings.

Use Mirilis action 50 gbs of video should be like 15-20 in mirilis same quality

My videos depend on what emulator I'm capturuing. snes games are literally less than a gig. n64 games are about 8 to 10 gigs and gamecube games are around 20. most PC/Mac games I play fall between 4 and 12 gigs. I use Screenflow.

If you can't figure vegas out in the end I recommend AVS Video Editor and AVS Audio Editor. Both great editing softwares and you shouldn't have that problem with them.

What version of Sony Vegas do you use?