For those who thought Donald Trump's Pope meme was bad

Too too much

Is that like one of the things here when they hire somebody to play the little kid on the show, and 20 years later he's still 4' tall?

Hard to say. There was a time early on when Su towered over Moa and Yui by about a foot.
Moa had a sort of growth spurt and nearly caught up to Su, but not quite. Momo, who replaced Yui, was about Su's height. I figure most people probably reach their max height sometime in their late teens or early twenties, but I could be wrong.

I just checked to make sure, because I was under the impression that we stopped growing growing around 18, but it turns out it's possible up to 25.

What they looked like in 2010, their first year. Su was about 13, the other two were 11. I first heard of them in 2015; by then they had become world famous.

Oh my gosh they're tiny! Especially the flanking two.

So those were the two that didn't sing initially, just danced?

They sang backup and sometimes sang duets to give Su a rest break. And sometimes Su sang solos to give them a break.