take a look at your VOB files - you'll notice all except the last one do not break the 1Gb limit. this is because of the way DVDs were designed, any file over that boundary is technically 'out of spec', and while it may play ok, some hardware may have trouble reading it.
however, if you're not burning it back on to a disc, it shouldn't be a problem to concatenate them. a quick google found me a few apps that would do it. 😉