The wind resistance shouldn’t be dependant on the mass. Shape of the sleigh would be the real factor.
But another thing to consider is that the gigantic mass and heat capacity. Given that the sleigh has a good heat distribution, it would take a lot of air resistance to actually make the sleigh combust. I don’t have a decent guess for the average heat capacity, so I don’t actually know if it’s significant enough, but the calculation is more complex than just looking at the speed.