Shrinking and growing then treads on Ant Man's toes I guess. It's possible they wanted her to have more of her own identity in the wake of many imitation heroes (Ironheart's coming up, new Hawkeye and Black Widow already arrived, etc).Frankly, I'd rather have two stretchy folks. It's not like there's a lack of "strong" or "agile" dudes. As long as you can visualize it in a way that is distinct (Kamala often does shrinking and being a giant, too) I don't feel I'd worry about that. The more silliness, the better, frankly.