Here's a thought. Bosses are resized based on the scenario.
Bowser is smaller when the idea is to sneak underneath him, but is much bigger when the object of the fight is different. Mario is a platformer in basically every aspect.
Ridley is big in general because the object of Metroid is to shoot ****, which would mean the smaller something is, the harder it is to hit. Metroid is a shooter/platforming hybrid.
However, this doesn't matter when it comes to Smash Bros. Why?
Because:
This game disregards all canon. If this game looked at "canon" or "realism" seriously...:
Mario nor Luigi could shoot fireballs without a fire flower
Mario couldn't hurt Bowser because in Mario games, you lose health if you touch him
Marth and Ike would be dead
Swords would kill characters
Olimar wouldn't be big enough to be visible
Mewtwo and/or Ganondorf would destroy basically everyone
Pokemon Trainer would be arrested for attacking people with his Pokemon
Ice Climbers couldn't use ice attacks. Because all they did in game was climb
Kirby would be tall enough to be underneath Mario's boot
Snake would use more guns in his moveset
R.O.B wouldn't be able to shoot lasers because he's an actual peripheral that didn't do much besides play games with you
I'm sure I could list others.
Also, as a video in here says, they're all TOYS! In Melee's intro, a kid grabs Mario, and chucks him into the air. The kid's hand is about the size of Mario. Ever see a toy that seemed really strangely sized? Well, I have an example.
Back in the good old days when I was really into Yu-Gi-Oh!, I wanted some of the toys that were out. This included a Yugi action figure, and eventually, an Exodia action figure. Well, in the series canon, Exodia is all powerful and towers above the player, right? Well this figure was only TWICE as tall as Yugi. Yea, weird right?
It's not all that different. Toys come in all shapes and sizes. If we took canon this seriously and didn't allow characters to do some of the things I listed, what fun would that be?
Have an imagination!