MIchaelW--Yes, I realize that there should be some knee flex, but I'm comparing my leg extension on the hybrid to what I have on my mountain bike, which seems to be a perfect fit. My legs extend very well on it with just a little knee flex on the downstroke; it's very comfortable, and I feel I'm getting good power from my legs, unlike on the hybrid. (FWIW, the mountain bike has a 16-in frame; don't know if direct comparisons of frame sizes makes sense here, since the geometries are different.)
I called the LBS; though he seemed surprised that the 15-in might be too small, he said that he has a 17.5-in in stock that I can try. If it is indeed a better fit, he'll make the exchange, so that's good. I'll just have to see if I think the 17.5 fits better or worse.