Yeah, I got my big Spotted Raphael Catfish after being told it got 3.5-4" (equated to a BN Pleco) when it was a tiny just over half inch ~ 1.5cm little baby. Don't trust the lfs
Now it is almost as big as my hand 4 inches across and about 5-6" tall and nearly 8 inches long with the mouth the size of a boulder marble (~1inch or like a oval quarter)! I see it wiggle out a few times a month to eat at night when I feed the bottom guys at light out.
The other one that is huge is the almost never see him upside down catfish (it came from my daughter who rescued it in a college town where someone was moving away and she took their fish - still looking for the person who has a shoal of these guys, to donate to, a year+ later). Its huge too. Big bioload little reward! I know what you mean.
Your tank is really looking nice though.