It's mostly RAM, which you have to set manually.
the biggest ones essentially crashed my game even with additional RAM--but maybe I should be more liberal with it, idk. Anyways, Engineer's Life 2 is a blast so I'm happily sticking to that for now.
Yeah, you want to set max RAM to at least six gigs(?) or more for some modpacks. Some of the leaner ones are fine at just 4 gigs, but its hard to tell if its enough, so just go crazy with it.
I didn't even realize that the RAM I gave it wasn't enough until I tried on my new gaming computer (instead of my old laptop) and realized that some modpacks not running at more than 5 frames per second and taking a half-hour to load wasn't cause my laptop was crap (it kinda is), it was because I didn't give the game enough RAM.
Note that there is a good chance your java is 32 bit and can't use more then 4 gigs of RAM no matter what settings you try to use. This isn't enough for some modpacks and you need to uninstall your java and get a 64 bit version of java that can handle it even if said modpack/launcher never tells you so or even implies anything of the sort.
Manually disabling some of the mods that you know you will never use any part of in a run can help a little bit too.