re: How did Hank Aaron hit so many homeruns being as small as he was?

I think they would as good now as they were then. While it's true that pitching has gotten better due to specialization, etc., they would have the advantage of using modern science to build their games as well (equipment, nutrition, modern metrics, new coaching, etc.). Most parks were bigger then than now. They'd have had the advantage of the youth programs that exist today.
Further, the NFL and NBA are far more popular now than they were back then...more athletes enter those sports, so, arguably, baseball doesn't have the cream of the crops of athletes that they did back in their day, so the competition is really thinner. Now this is offset by overall population growth to some extent.