As you can see i.e. in judo or Olympic wrestling, there are the weight categories. The reason is simple: more there is a body-to-body contact, more weight counts.
It was not always that way in judo, though. See Aikido Journal's articles on Yoshio Sugino. One of the tiniest men ever, he was incredibly formidable in judo and he strenuously opposed weight classes in judo because he (like Mifune) would take on anyone and usually won.
He quit judo when they instituted weight classes because he felt that took something important from the experience and thereby changed the basic nature of judo.
I think the same thing applies to aikido. Relying on extra weight to effect techniques or allowing an attacker's extra weight to limit your technique are both failures of understanding.
And as for "getting under" the attacker, that can, in fact, be done when your hips are higher than the opponent's. That's a lot of what the internal strength discussions are about and why modern technical aikido and judo fall far short of the arts of Takeda, Ueshiba, Mifune and Sugino.