For me it's a combination of many factors.
I think it's in part associative. I like it when women wear athletic gear. "Sexy" outfits are generally less sexy on these women, not because of a poor fit or anything, it just detracts from a major aspect of the attractivity.
The performance aspect, with the sweat and the effort, I find attractive. Some old and forgotten associations (PE, sports situations) with early experiences of women's bodies and their smells I guess.
There is an appreciation of high strength for sure. When I see someone like Loredana Toma moving heavy weights with extreme speed, I can't help but find that very attractive.
But the extreme mental strength I know it requires is also attractive (extreme ability is often attractive in other domains too, like arts, music etc.).
And then the aesthetics of a very fit body, it seems to trigger in part the 'normal' side that likes curves (mostly when it comes to the legs), and on the other hand perhaps some sort of unchecked (but fundamentally natural) appreciation of fitness.
Perhaps it is not that we have some 'thing', perhaps it is that the normal situation has the presence of some instinctual sensation that a woman who is very brawny has too much T, or is a threat?
I don't think non-muscular women are unattractive, but I do think I have more of a problem than most with e.g. thin limbs. I find that it seems frail, which - all else being equal - feels unattractive.
It might be in part genetic. I am naturally quite lean and find it relatively easy to build muscle, so maybe that's an expression of having descended from other males that choose fit women. Who knows.
As for women being stronger than me, it's completely irrelevant to my attraction, except that I want to be as strong as possible to 'deserve' being with a strong woman.