So I thought, does being heavier (larger centre) improve one's Aikido?
Due to greater ability to keep weight underside and being more balanced/stable.
Imho power to weight ratio is important. If one is bigger with the requisite core and lower body muscle to move the weight effortlessly then any negative effects may not be immediately felt. But bring in other aspects such as cardiovascular fitness, endurance, flexibility etc. then being too large can definitely have a downside or two.
As for me, I've found that losing 15 lbs recently has increased my speed, neuromuscular response time, overall coordination and precision of technique. In full tanto randori/shiai the difference in time and space because of size can have a serious effect imho. Since timing is critical, that change in weight/size can be the difference between executing effective waza or being stabbed with a rubber knife.
To me, the weight can be advantageous in executing certain types of waza but if it hampers ones tai sabaki too much it may not matter.
My 2 cents.