Movie releases by major studios are typically on pressed (not "burnt") dual-layer DVDs. What we have available as "recordables" are DVD5 (4.37GB) and DVD9 (7.95GB), also called R-DL, the DL meaning dual-layer. The DLs are still expensive, but you can indeed do a straight copy with DVDDecrypter, for instance. ISO Read/ISO Write.
You have to decide how to approach this. Money is no object? Burn DLs. Or perhaps you don't have money to "burn"? From here you have at least three ways to go:
1) Reauthor main movie only, keeping only one audio track. This will mean best possible video quality, often requiring no compression at all.
2) Full backup. So you really think those nifty menus and extras are indispensible? [shrugs] Suit yourself. Picture quality will be degraded, to what extent it will be noticeable depends greatly on your display.
3) Full backup, but post-process selective "blanking" of unwanted extras to gain space. VobBlanker and TitlesetBlanker are two examples of apps used for this.