Hi--I'm new to Piano Street, and am very interested in the concept of "finger strength." But technically speaking there really is no such thing--check out any anatomy diagram or ask any doctor, and you'll learn that THERE ARE NO MUSCLES IN THE FINGERS--only in the hand and arm. (Sorry if someone already wrote this and I missed it.) The fingers are complexly controlled by minutely coordinated movements of finger and hand TENDONS that connect the contracting/relaxing hand (and arm) muscles to the finger bones. (I'm not sure I'm explaining this optimally; sorry.) But this fact raises important questions about how pianists develop fine, sensitive, and yes--strong-when-needed--touch. The "strength" ultimately comes from finely controlled coordination. Over time, many pianists do develop noticeably enlarged muscles under their forearms near the wrists, and probably at the bases of thumbs and pinkies.
I also think that "fat-fingered" people may well have an advantage over thinner-fingered ones, but maybe mostly because the fingers are probably shorter more than fatter and therefore do not need to bend back away from the backboard, particularly during octaves and wider stretches. But it's people like Martha Argerich who I think were really born to play--check out early videos; she was quite thin then but her arms were reminiscent of Popeye's. I think those heavy, strong arms gave her an advantage over many. (Oh, and maybe her thick, short-looking fingers? Just my 2 cents.)
"Strength," though, is definitely an aspect of powerful playing, but not always the most important one. I think this topic could be explored in many ways.
Adieu for now...