Mage boxing is around 3/4 of your defense to magic according to Jagex, and about 20% or so your actual defense level. If you're having trouble hitting them, use blood/ice blitz, more accurate.
I have a lvl 123, 89 def, 95 hp and 99 mage and I've beaten plenty of maxed guys. Defence doesn't matter too much but it does matter, you just got unlucky.
Don't think you should win every stake, that's not the right attitude to have because you WILL go on a losing streak at one point. Your stats aren't at fault.