Im capturing Laurel & Hardy from my digitaly restored video collection and i have a question.

Why is it thet when i capture black and white video the file sizes seem to be much larger than capturing a 2 hour film (colour) off digital tv, and also when i come to compress it i have to use lower quality settings to keep the file sizes smaller?

Do i have to capture black and white films in a different way as im not sure why a 65 minute black and white film captured from vhs would be 5 gig captured with xvid highest quality yet a 2 hour colour digital film will be 4 gig.