I'd say you're fine to increase up to 2 cups. You don't have to do it all at once, but spreading it out over 2 weeks or a month should be more than enough. I've had poopy-butt issues with feeding new greens to one of my rabbits (the other has an iron stomach) but changing the amounts has never caused problems.
At her age, I wouldn't decrease pellets (she could even be getting more, according to HRS guidelines). That brand of pellets certainly isn't so rich that she can't have a bit more as she's still growing. I would switch to a timothy based pellet once she's 8 months to a year old.
She looks like a holland lop from your siggy? Holland size can vary from 2-4lbs (and some even a bit larger if they are false dwarfs) so being small isn't an indication that anything's wrong with her. She's probably just a smaller bunny. My holland boy was full weight at 6 months, so she probably won't get much bigger.