It's all about the minimum "thermally addressable" unit of capacity! Smaller cell size helps with homogeneity a lot..... aluminum casing on the cells conducts heat decently well. Pouch cells don't have a casing to conduct the heat. Also 1/3rd of the cylinder surface in contact with the cooling plates is significantly more than the Bolt.
A 2170 cell has ~4600mm^2 of surface area along the sides. ~7.15 square inches.3000 cells - 21,450 sqin of surface area. One third - let's call it 7,000sqin.
I have some Model 3 cells with me now and it's pretty clear the casings are nickel/steel, not aluminum, given that they're ferromagnetic.