Fall Down, Go Boom
The head instructor at my first school was a woman, and I easily outweighed her by a hundred pounds. But, she had no problems bouncing me off the mat like a basketball.
However, at that time, I was a brand-new white belt who didn't have any previous martial arts experience. She was (is) a sandan.
On the other hand, I have a friend with whom I currently train who is significantly smaller than me. (I'm 6'1" 250 lbs. and she's 5'2" 100 lbs.) We've been training for roughly about the same amount of time, except that she is slightly senior to me. Now all that being said, she is convinced that she could easily handle any committed attack I could give her. Personally, I don't think her aiki is sufficient yet to where she could "put me down" if I really intended to do her harm.
So all that being said, I think that a very good, accomplished aikidoka is capable of defending him/herself from a larger opponent. But I also think that there are variables that come into play: relative levels of martial ability, awareness, physical fitness, and (if all other things are equal) size.
Just my ramblings on the matter.
Eric Schmidt
(Yay! My first post! Woo-hoo!)
Eric Schmidt
Student
Aikido Heiwa
Lynnwood, WA