My problem is, I have size 15 feet, and am diabetic, with the attendant poor circulation and neuropathy of the feet.  I thought my feet were somehow staying warmer as I got older, but I just can't  feel them getting cold...not a good thing on a sub zero day!
I have tried liner socks under heavy wool, which did not do the job...ever, even in my X-county ski boots.  They crowded my feet in my former boots, which I thought should be 13-14s. 
I now have some Cabela's Iron Ridge boots,1000gr. insulation,  and wear a heavy pair of  wool socks with them.  Plenty of room, and my feet seem to stay very warm.  Maybe not the best, but they are what I can afford, and come in my size, which is no small thing.  No pun intended.  

I have a pair of Browning boot socks, and another pair of Smartwool socks, which are very heavy, and  my size, that I will be trying out this winter.