That's not for strength - that's to reduce the likelihood of your boys interacting with the top tubes! An equilateral triangle is the strongest geometry. The more you flatten it the weaker it gets. The triangles are NOT smaller - well it depends on how you define "smaller", but the length is the same to fit a person. A small triagle would be 3" x 3" x 3" and yes very strong, but kinda hard to ride. Compacts are just flatter. And flatter is not stronger. If it were, you'd dispense with the seat tube and top tube altogether. It'd be infinitely flat and therefore infinitely strong. That's not how it works.