Although now that I think about it, the difference can't be too big, since in Return of Harmony 2, Twi was able to keep AJ pinned with her body weight.
Thinking on the other ponies, since Dashie can keep pace with AJ, even in a full on brawl, I'd imagine should wouldn't be too much smaller than AJ. Either that or if she is quite a bit smaller, she'd be super toned.
Going off of that, Pinkie gave Dashie some issues in getting her over to the barn for her party, so as far as size and strength goes, I don't think she's a slouch.
I'd wager Fluttershy is much stronger than she lets on as well, given what we saw in Return of Harmony 2. I'd probably guess she's one of those types who would look kind of skinny and fragile, but is actually all muscle underneath. She's just too shy to put all of it to use unless extremely agitated.
EDIT: Forgot to mention Dashie carried four ponies at the same time in Rainboom. That's some serious strength there.
edited 12th Oct '11 1:53:13 PM by KuroiTsubasaTenshi