it could be. its usually compression of the dorsal pedis artery that causes this
you could adjust where your ankle strap is as well, when its tight it should be even over your foot, not way off to one side
Doc is a doc and he's going straight to the physical answer. Correct me if I'm wrong, but my understanding is that it takes like only 7 pounds of pressure (per square inch I'm assuming?) to start cutting off the circulation to that sucker.
From a bootfitter's perspective, my immediate reaction since you say that it's not your instep strap being too tight is that your instep of your foot itself (the "top" of your foot where the nerves and blood vessels are) is pressing up against your boot too much. Click here for a good read on why a proper insole and/or matching up your naturally high instep with a high volume boot could potentially be better.
Sure packing out and breaking in will accomplish some of this, but I'd have a good bootfitter check out your foot and your insole situation if you have one anywhere in your area. If not, email me / PM me and I'll walk you through some things you can do yourself to test a theory or two......
And yes to thinner socks