[Power Girl] was created in the seventies as an attempt at a "strong female persona" (her personality, not her muscles). originally, her breasts weren't as big as they're generally depicted today.
What it boiled down to was that Power Girl was, for all intents and purposes, originally drawn by Wally Wood (the first couple of issues had Ric Estrada layouts, but for all intents and purposes it was Wood's art). Wally Wood is widely recognized as one of the masters of comic book artwork. He's also famous for busty female heroines and, as a result, the sole female member of the mid-70s JSA had huge knockers. Wood was also sort of an odd guy so I wouldn't doubt he slowly made 'em bigger.
Later artists on the book (primarly Joe Staton) didn't draw her so chesty. But the impression of her being especially well-endowed lingered with readers of Wood's run (I know it did for me). This is what probably led Sears and others to start making it a character trait.
I remember it being much smaller in the JLE era, they even covered her legs. It was JSA were she got back in the bathing suit and the circle was made larger.
