The commercial DVD's are usually 8.7 gigs because they include
both wide screen and standard screen versions of the same movie
plus all the extra menus.

No... The commercial DVD's are 8.7 gigs because they use dual layering
technology

? No to what ? It doesn't matter how they get 8.7 gigs. The
fact remains that if a movie uses the full 8.7 gigs it's usually
(not always) because there are two complete copies of the
movie on the disc.

that most home burner units can't do. Some movies are more
than the 4.7 gig and they have only one format on the disc.

It all depends on the original encoding of the disc...

Dual layering is most often used to include two movie formats,
therefore most movies can be copied without any quality loss.