Let us not forget that ftb adds literally hundreds of new things to build, many of which require diamonds.
In vanilla, diamonds go towards what? Tools? Armor? ... An enchanting table? In ftb, many recipes have their usefulness counter-balanced with a weighted cost in resources. I've been re-wiring my base with that IC2 glass-fiber stuff that costs a diamond for half a dozen lengths of it. Half of the applied energistics stuff has a diamond thrown into the middle just to make it expensive. Wanna build solar panels or modular power suit stuff or just about anything else that's expensive? Diamonds, mate. And I'm not even complaining that some of the recipes are or not over-powered/price-inflated/whatever. Take it or leave it, the long and the short of it is that there's WAY more stuff to do with your diamonds now than in vanilla, so finding more/bigger veins of them could, arguably, simply be to balance out the crazy amount of new recipes.
Diamonds are also no longer the rarest items in the game. Uranium can be a pain to get a hold of, depending on what you're doing. Iridium is the rarest thing in the game, and in unleashed is only obtainable by dedicating a chunk of your infrastructure to it. And some of those bees? Fogetabboutit. Diamonds? Useful and kinda rare, but no longer the only metric for "how rich are you in minecraft."
So in conclusion - are diamond veins bigger as a result of some confit setting or mod? Maybe. Probably. I don't know. Am I glad if that's the case? Youbetcha. Scouring the landscape for caves to dig up exposed uranium is already a big enough pain. Doing the same for diamonds might be the fun/hard mode that some folks like, but for me...I'll stick with less grinding and more ...whatever it is I do when I play.
I completely agree.
In Vanilla I have so many diamonds Before I even get a silk touch pick that I never have to Fortune 3 a single one of the many many diamond ores that I Silk touch. If FTB I can burn through all of those and then have to go through all the ores and still can find myself short on them at times.
Even emeralds are harder to find than diamonds, outside of farming villagers for them, in vanilla.