Who said anything about stealing anything? Rules can't be copyrighted and how long do you think it would take for crib notes to end up at alternityrpg.net? It's all perfectly legal, I couldn't care less about the filler.
And it would pretty much double the size because we aren't talking about a campaign book, but a rule book. A rule book that presents two completely different sets of rules would be about double the size. Do you think you could fit both the Dungeons and Dragons PHB and WHRPG into one book without about doubling the size? Take a look at the size of the d20 SRD and compare it to the size of the complete PHB, there's not as much difference as you'd expect.
And it would pretty much double the size because we aren't talking about a campaign book, but a rule book. A rule book that presents two completely different sets of rules would be about double the size. Do you think you could fit both the Dungeons and Dragons PHB and WHRPG into one book without about doubling the size? Take a look at the size of the d20 SRD and compare it to the size of the complete PHB, there's not as much difference as you'd expect.