That's not the impression I'd got.
If we look at the companions:
Shadowheart has been memory-wiped by Sharrans, that could explain why she doesn't recall how to use most of her powers if she was ever more powerful than she is now (which we don't know, though if she has some connection to Moonhaven then she might be significantly older than she looks and so it would make sense).
Lae'zel is implied to be pretty young and experienced anyway,.
Wyll is implied to have been pretty useless pre-Mizora, so with her captured and unable to help him it makes sense he's no longer powerful. Or perhaps she withdrew her powers anyway after the falling out they'd had just prior to capture.
Astarion has been kept as a lowly puppet of a master vampire for a couple of centuries. It's not clear if or how he'd have gained power in that time, and prior to that he was a magistrate which doesn't necessarily mean he was experienced in any particular class skills.
Which is not to say you’re not right, of course. There could easily be some clue that it was definitely tadpoles that I’d missed!
I had the same impression. I think Gale is the only one who is implied to have once had significantly more power than he has now. Wyll mentions that he is now weaker than before, but I also think this could be due to Mizora being captured.
