DVDRebuilder making movie too small, Any ideas

I am just starting to use DVDRebuilder using QuEnc & have made a few discs without any problems until now.
When encoding dvd “Pooh’s Heffalump Movie” in it’s entire whole, DVDrebuilder shrinks it from around 5.5Gb to 2.5Gb NOT the 4.5Gb I need.
Can anyone explane this, please.
There is plenty of space on my harddrive
I have used uncompressed files created from both DVD Decrypter and DVD Shrink for DVDrebuilder with the same results
Any help would be greatfully received.

disney dvds are often structured in complicated manners, which could throw off the bitrate calculations for transcoders/reencoders. maybe there are others who are having the same problem as you who can add to this.

and can you play the compressed movie? although i have read about these problems with size, never had it. but i´m using cce or hc encoder. try to read this