It's probably just because the fabric has stretched a little. If it's just a tiny bit, I wouldn't worry about it too much, most people's joints fudge once in awhile. Just tug it a bit and pin.
BTW, small is not easier. I did some Bear Paw blocks that the finished center sq was 1/2" and the finished "toes" were 1" HSTs. They were hateful little things and if it had been something for myself I'd have tossed it.