Compared to what?
I was concerned too. I am currently wearing a Omega Speedmaster Professional which is also 42" diameter, but the case is 14mm. Being the Apple is 10.5, I am actually getting a slimmer watch now! I think it's a matter of what you are using to wearing...
No. 10mm is not thick at all for a watch. My Breitling is like 18mm thick. I would wager that the vast majority of mechanical watches are over 10mm thick.
It's BBW.