These liners are a good deal thicker than socks, but thinner than most heavy felt pack boot liners. They are about a quarter of an inch thick, and much bulkier than socks. They are contoured to fit in the boots (you might be able to put them in a different pair, but only if you are replacing a different liner or the boot shell is at least a size too big.
I would say medium thickness. I wear thin socks with the boots and my feet stay warm and dry. I have been wearing them this week in Chicago on a work trip and I have no complaints.