Superman has always been drawn muscular in the comics, not to the extent of the Hulk, but he's always been bigger than say Spiderman or Batman. Superman is popular among bodybuilders because he is the most exposed. Superman is popular with the general public because everyone knows who he is. He is not only the most powerful, he is arguably the most popular fictional character of all time.
He was not only the first superhero in comics, but the first in Radio and Cartoons. Superman was one of the first superhereos to have a movie serial, and the first to have a feature film, which was like a pilot for what would be the first superhero television show.
Superman also had so many different comics featuring him, that if you read them as a kid, you were bound to be exposed to him. Lee Priest never really read the comics, but always watched the George Reeves' show, and loved the character from that.
Bodybuilding is about how you look, not how strong you are. But when we start out, every guy not only ants to get bigger, but stronger too. And who is stronger than Superman?
And as for his powers, originally Superman had tremendous strength, his skin could not be penetrated by a bursting shell, and could leap great lenghts and heights. This was all attributed to the fact of Earth's lighter gravity than Krypton's, by his creaters Jerry Siegal and Joe Shuster. When Superman started to develop all the vision and eyesight powers, these were due to the Earth's yellow sun .
So everyone is right to an extent.
