actually theres a pretty neat trick you could try, render it using these settings:
(i have vegas 11 so if yours is different my apologies)
1st: click render as…
2nd: choose “Sony AVC/MVC (mp4.mt2s.avc)
3rd: (the tricky part)
Video Format: AVC, Resolution: 1920×1080, Entropy Coding: CABAC, Frame Rate: 25 or 29 fps whichever you like both work for me, pixel aspect ratio: 1.0000, bit rate: 2,000,000
Audio: Format: AAC, Sample: 48,000, Bit Rate: 192,000
System: Format: MP4
Project: Video Rendering Quality: Best
these settings should give you an HD video with low size, most of the time it works for me HOWEVER its highly dependant on the source it comes from, if your video is HD it’ll come out in HD, give it a try and see if it works