I believe topic is not about being overweight.
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. In aikido we have far less contact, but still, some static techniques requires a lot of contact to unbalance attacker. Particularly we can see it in front or rear both hands grab attacks, or chokes. As not everyone is able to create a dynamism even from static attacks, as long you are not doing 'aikido before contact' (TM), you are in disadvantage compared to equally trained heavier attacker.
Good point but I think then it really does not matter. The point goes to the person with the most skill. And at least form what I have seen so far the person who can stay the most relaxed and centered and does the least seems to be the most effective. At least, I've noticed, that against someone of equal or greater training level using my weight and power becomes a big disadvantage most of the time. As uke he harder I come at them the farther I fly and as Nage if I try to muscle them I generally wind up giving them something to use against me. Sure sometimes I can force it with strength and weight, but then its not aikido anymore.