How much does the thickness of a pair of socks play into how a shoe fits? Could it be up to a 1/2 size difference between thicker athletic socks and thin socks/barefoot?
I bought a pair of Kinvara 3’s recently. I tried them on with the normal running socks that i would wear and they seemed to fit fine. When i wear thinner socks or no socks there is a lot of room in the shoe and my feet slide a lot in the shoe.
It can easily be a 1/2+ size between really thin racing socks and thick cotton crew socks. Like you, I wear my running socks when trying on shoes (I tend to like really thin socks though). When I wear them with regular crew socks, I have to loosen the laces.
If you want to see the volume difference it can make ball up a very thin sock (Feetures Elite Ultra Light, Swiftwick Aspire) in you hand and compare it to a think sock (Balega Hidden Comfort is a little thick, Drymax “run” socks are the thickest) It makes a difference.