Eh, you don't really need that much RAM. Probably if you're using 200 mods you'll want 2 GBs, or 1 and a half, but 3 is too much. It's because of the 'garbage disposal' of java, which frees up the RAM.
Short way: Many RAM is inefficient.
Long way: The garbage disposer of java frees unused memory. It's what when press F3, what makes the used RAM go down. if you allocate a lot of it, a good example is this:
You have 100 pants, and use only 10. See the problem? There's 90 pants in your wardrobe making bulk.
Just leave it with 1 GB to start with, then add some more if you feel like it needs more. If the ram goes from 50% to 100%, it's the garbage disposal not working properly, not the lack of RAM (it's like you're waiting till the last day to do the laundry and have to clean 50 pants!).
That's the worst lie I've ever, ever heard.