The only characterization he needs is that he's the central bad guy and the most important evil character in each and every game. Whereas, contrasting this to Ridley, he's present in a two boss battles,his pirates are there, but he's never the most important foe. He's relevant within the series for stealing the Metroids, but that happened 27 years ago. At this point the game is more recognized for DESTROYING MUTANT ALIENS WHICH R DESTROYING PLANETS AND STUFF than any Ridleys original contributions.
None of anything which you've said takes away from the fact that Ridley, at this point, is thrown in to pay homage to the old games, rather than display his own relevancy. Yes the pirates creating Prime in that series, but in that role the pirates are the catalysts behind the genesis of the problem and aren't the problem themselves. Their relegated to roles of inconvenience, as Ridley is.
And I wasn't saying scale was important for anything within the game series. I understand how the characters got it the games. But while they're in the games they maintain a level of consistency throughout the series. They normally make up X portion of a platform and are comparable to one another. Ridley is big as fuck compared to the others, as indicated in Brawl, so why would they change that. I'm not saying that they would never change his ration to input him as a character, but its doubtful.