Guys are phsyically different to girls, different muscle strengths, densities and explosive power. A 70kg guy in peak form and a 70kg girl in peak form are going to have different kinds of strengths. For that matter there are racial differences in muscle compostition and then individual differences too. As a general rule testosterone assists with explosive power and generaly females have much lower levels. A female with high testosterone levels could well be stronger than a male with low levels. But its not just what you have its how you use it. Isnt the point of this all that you should have a series of sparring partners with different strengths so you are better prepared for what may come? In an ideal world you should be able to automaticly adjust if your opponant is fast, strong, has particularly good kicks, etc. i thought sparring was about tuning your reflexes and responding to your opponants cues not beating the crap out of eachother no matter your gender. Does it matter who your partner is if your padding out your skill base? and if your worried about getting hurt is this really the right sport for you? I would just as happily spar with a male or female, someone bigger or samller than me, it can only make me better.