The three day limit is agonizing! grrr. =) I've been reading a lot of the posts on IC and a common complaint is that IC undersizes images.
Whereas it will report 4.31 or whatever, and the final image can end up being much smaller.

I don't have the name of the individual handy (thank you for bringing the issue to light) but someone started a thread in regards to Back to the Future. That the results were about 8% other then what was reported.
I can verify that for images that are not at a full 100% (perfect, no transcoding)
However once you do ANY transcoding, IC seems to be undersized.

With that said..

I tried backing up Shiri (a really pimpdaddy Korean movie) and after removing the extras, excess subtitles/language tracks I got a 94.5% movie quality. So going through the whole transcode process, I get a file thats 3.91 gigs. That is painful. You would figure that another 5.5 percent would on the movie wouldnt be .79 gigs ( or 790 megabytes ) larger.
So I try again.. I inspect the size of the main vobs (the main title) and that comes out to 4.52 gigs. I then retranscode it in IC with the quality set to 100 percent. I hit F5 to override the size limit and then transcode to HD.
When I was done, the PDI files were 4.52 gigs in size.

This leads me to believe that Instant Copy's compression algorithm differs from movie to movie. While I am sure that if I retranscode the movie and increase the size 8% it would still work, I feel that 94.5 percent quality at 3.91 gigs is acceptable.

While you can fit 790 megabytes worth of extras (significant) people are interested in at least the main movie only can take comfort if the image comes out below the maximum disc size.