I remember a video being posted here many years ago of a 300lb+ person going into a rampage in a store and everyone here was wondering how they could possibly deal with someone like that with their Aikido. The answer is quite clear, get strong! If you can squat 400+ pounds and deadlift 500+ pounds, doing a single leg takedown, or a variation of koshinage, of a guy that big would be pretty easy actually.
Two problems with this.
One is that, for a woman my size, 400# is north of the world record squat. Not happening in this lifetime. It's more achievable for men, but still is going to require a good amount of gym time. And, unless you also weigh 300 pounds, you're still not going to be as strong as Mr. Berserker. (Who, it's safe to assume, is also under some kind of chemical influence.)
Which brings us to problem number two, which is that training like a powerlifter is not really compatible with the flexibility and sensitivity that aikido requires.
Certainly it doesn't hurt to be strong. But saying that strength is the answer to whatever might be wrong with your aikido is simply missing the point of how aikido techniques are supposed to work.
For the record, I've successfully koshi-ed some pretty big guys. If your koshinage depends on strength, you're doing it wrong.