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.
I'm well aware the weight categories are recent innovation. But they were introduced for important reasons. I agree also there are some very exceptional artists that are able to neutralize their own disadvantage when attacker is much heavier. However, in daily training, for us, mortals, as I already wrote "you are in disadvantage compared to equally trained
I don't see it as "limiting my technique". On contrary, it forces me to enrich my technique every time I deal with somebody 100 pounds heavier than me(and that really happens regularly). But I have no illusions what can be done and what not in the dojo environment where you protect an attacker. I saw ppl who like you didn't respect that and got seriously injured by their own ego (not intentionally by attacker!!!). You can't cheat gravity.
I have average weight but as a uke I can be very heavy and redistribute a power of the aikido technique to degree zero. If you give me 100 pounds more it becomes scary to think how I can use this additional weight…
Also I tend to separate the legends from reality of training.