welcome to the wonderful world of compression
a dvd fits 4.4 gigs, estimate...
iDVD compressses max. 120min of a .dv-stream into that.
this is a compromise of quality and compression.
with a pro tool as compressor etc., you could get a higher quality or more compression.
e.g.: your whole movie is a plain green screen (ok, that is not REALLY a movie

)
you could compress that for hours and it woudl fit on a cd.rom...-
the socalled delta (difference between two frames) makes the size of a comperessed movie...-
so, don't say a dvd has 120min...- it's 4.4.gb