It is surprising just how much stronger men are in upper body strength than women. I stress, lest I be lynched, that I refer to upper body strength, not any other physical or moral quality.
This post brings the data (pdf link, to paper from European Journal of Applied Physiology)
http://egitim.judo.gov.tr/Dosyalar/makaleler/-ENG-Hand-grip-strength-in-judo.pdf
Takeaways:
"the very strongest female athletes are barely above the median of grip strength for men.
The top 75th percentile of female athletes are below the bottom 25th percentile of men.
Ten percent of women have stronger grips than the bottom five percent of men.
The very weakest male in the data set of nearly 1,700 males looks to be about at the 20th percentile for average women."
This from a study of grip strength, n=1654 men, 533 women (including trained athletes and untrained people).
There you have it. Frankly, a small guy can almost certainly beat a heftier-looking woman at arm-wrestling, due to nature having endowed men with about twice as much muscle mass in the upper body.
None of these facts needs make any difference to your Hero characters, if you don't wish it, of course.