Bruce Lee himself said "a stronger muscle is a larger muscle". The world's strongest dudes are also some of the largest dudes. The guy that plays Gregor Clegane is 6'9" and weighs over 400lbs (according to google. Also, he's a good example of a guy with more than 18 str, and I'd say he qualifies for powerful build).

So, no, you will never find someone like emma watson who has a str of 11+. Han could maybe hide 14 str under his clothes.

I've known people who were surprisingly strong in some ways, but it wasnt meaningful enough to affect their str scores.

Now, constitution plays a role. Say you have two 15 str guys, where one has 10 con and the other has 14; I'd bet that the 14 con guy is heavier/larger. So, an elf with 10 str will probably weigh less than a human with 10 str, but your 17 str 90lb elf chick is unrealistic.

All said, I don't care if somebody comes to my table with a 17str 90lb elf chick (I might mention it or occasionally make fun of the fact); it's a game, and a fantasy one at that, where dragons can fly.