Because of the way he's shaped, he tends to retract into that standing position as posted above, but when he attacks, he has ridiculous range.
If you made his height about even with the tallest character, the fact that he's retracted makes him de facto bigger. To compensate, they'd have to make him even smaller.
Unless you did actually make him really tiny (probably tinier or weaker than Charizard due to Charizard being stumpier and less like a coiled spring), he's going to have a lot of range.
Knowing this, you could make him slower, make his attacks weaker, or make him weigh next to nothing (and therefore get knocked around easily) to balance him.
Ridley by nature is none of these things, and while you could certainly do all of the above in moderate amounts, what you'd end up with is a creature that doesn't seem to move at all like Ridley moves.
So what do you do? Increase the speed and nerf his attacks and weight even more? Then you end up with a Ridley that looks good, has decent range and speed, but plays like Marth or Shiek, barely able to hit hard. You'd have pansy Ridley.
So forget that. Let's make him slower, but have his attacks hit pretty hard.
Then you're back to him not looking right in his movement.
Bowser and Ganondorf were both already more balanced in their games. They were heavy/slow/strong.
I'm not saying Ridley is too good because of Metroid's popularity. Ridley is exceptional because that is how he is portrayed in the Metroid series- heavy/fast/strong. It isn't the type of villain you see in everyday games. Most games innately fall into a more typical villain set up.
And yes, I'm mostly basing this on Prime considering in Super Metroid he doesn't really move his body at all. He just floats back and forth shooting fire and tailing, etc.