lsd & different tire sizes

[Qdog]

Checking the air in my Michelin Arctic Alpins, I noticed it said 100 load rating. Then I noticed the size was rated 225/70/15, funny I thought I had 215/65 15's on the car. Well the other three are 215/65, one is odd ! It's on the rear, for the third winter now, how did I not notice this ? They came with the car and they're wearing very well, by the way. So I have different diameter tires on the rear axel. What is this doing to the lsd diff ? Am I wearing it out with this set-up ? This is Minnesota, so I have 6 or 8 weeks before I can put on the summer tires. I don't think I'll use these for another winter, but have I done any harm to the diff ?