I think Snake weighs so much because he's the only "realistic" character there is. So, he's given weight to match what you'd expect from a 200+ lb. muscular soldier, as opposed to Marth and Ike, who come from a more fantasy setting and are more anime in their appearance. I like it, I think it reflects how much he stands out compared to the rest of the cast in terms of aesthetic and source material. It also ironically adds to the wackiness a bit by having this realistic dude be in the same weight class as Bowser and DK.
King Dedede is heavy because he's supposed to be the big gluttonous king of Dreamland. It's less about what's realistic here and more about what reflects the character. He's such a glutton that several of his conflicts with Kirby revolve around him stealing food. It makes sense for that kind of character to be heavy, and would be odd if he wasn't.
Bowser's heavy for a similar reason. He's the big bad of Mario, so making him the heaviest reflects his reputation. Of course, he's also hard and scaly, with a big shell, so that all adds to his weight more than most other large fighters.
I'm pretty sure Ridley has to be heavier than Charizard. Charizard is apparently 5'7" and about 200lbs, which is about the height and weight of an average adult man. Ridley's a huge space dragon, albeit scaled down to be playable in Smash. Sakurai's concern for Ridley's playability has always been about getting that essence of a huge boss character into a balanced state. So it's more likely that Ridley will be on the heavier side to capture the essence of his boss origins.