Well, at "best quality", Studio should give you a bit more than 2 hours on a disc, so right off the bat, best-case, you're only filling 3/4 of the disc.
Add in the fact that Studio does seem, even on the "busiest" of video, to seem to undershoot by about 10-12%. This is likely a cross between size estimation being pessimistic, plus VBR encoding sometimes yielding an overall lower bitrate than that specified if the video is "clean" (not a lot of low-light "sparklies", not a lot of movement, etc.)
Furthermore, if you're doing background rendering to DVD-compatible MPEG2, anything that Studio background-renders during edit may not be re-rendered during make, and may be written out at the background-render bitrate -- which I *believe* is 6000 kbit/sec. 6600 kbit/sec will yield about 180 minutes of video, which would mean that your 90 minutes would only about half-fill a disk (worst-case scenario).
So--another possiblity: In your case, you should be creating an approx 3.25 GB image, if everything is working optimally. However, Studio will usually create an image that is up to 10% smaller than that specified (worst-case scenario), so in your case that would be around 2.9GB. AND, if you have any background-rendered sequences that have been rendered at 6000 kbit/sec, that would further reduce the size of your disc image.
I could see you legitimately getting down to 2.26GB even when rendering video, if you have quite a bit of background-rendered stuff.
And if that's the case, one way you could test: Turn off background rendering, delete Auxiliary files, and create another disc image.
And a 3rd possibility: If you're capturing in native MPEG2 (such as from an MPEG2-capturing Dazzle device), and you're capturing video at the standard "DVD compatible" 6000 kbit/sec, then 90 minutes of DVD video would end up being pretty close to 2.2GB *regardless* of output settings, as long as the target bitrate for your disc is higher than your capture bitrate. In this case, Studio may "smartrender" the input video directly to the output video without re-encoding (and without further dropping the video quality). So...that's another way for a disc to not be "filled up", but to still have good quality.
Hey, none of this may be "the" answer, but these are scenarios that MIGHT help make sense out of this observation, anyhow...
BittMann