Aww they're beautiful. They definately decide in one day they need to sprout much bigger. Cleo did that, she went from tiny to hard to differ from Timmy who is twice her size. It's amazing how they change
Gizmo, who was the runt of the kitten litter we took in, is now big enough (and, unfortunately, smart enough) to stretch himself up and open the screen door onto the front porch! First time he did that, I was like "when did he get that big???" I still think of him and his sisters and the "kittens" I think they just hide it from us until it is in their best interest.
Yes, I read years ago that human kids (at least) really do grow in spurts, suddenly shooting up an eighth of an inch in a single night, for example. That's what causes "growing pains" -- those sudden changes!