Of our two remaining kittens, the girl-cat is likely to grow up to be about her mother's size

Small, but not freakishly so like her sister. I expect her to stop growing around September, maybe a little later but not much. She might get bigger than her mother simply because she's not going to get pregnant at any point. At this age, of course, you can't really see the growth from week to week.

Her brother is something else. He dwarfs his sister, and probably his other littermates though they're not here to compare. He outstripped his mother already, about a week ago. (In length. He's weighed more than her for a few months now.) And I know exactly when this happened because the week before he was still smaller than she was. He's 8 months old and you can still see visible growth in him on a weekly basis. And larger animals take longer to grow up, I wouldn't be at all surprised if he's still getting bigger (in terms of bones) at 18 months or even 2 years. (Muscle mass takes a little longer, and they'll still be putting that on for some time after they cease getting longer. Their mother's small size in that regard is one reason I suspect she's a very young cat after all.)

It's a little frightening, actually. And to make it worse, he's not a subtle creature at all. Some cats, you know, they have the art of making themselves smaller than they are. Not this guy. He doesn't rest, he lounges. He sprawls. Like some humans, he's determined to take up as much space as possible in any given instant. Even if he curls up he's always careful to stretch his front paws as far in front of him as possible. He's going to be huge, he already is, and his goal is to make himself appear even larger. It's like he knows this is his huge tactical advantage in all situations and he's determined to take advantage of it even when it's completely unnecessary.

I truly marvel. I'd take a picture, but I'm not even sure what I'd use as a basis of comparison.