When you consider the recent lookbooks, and the fact Supreme gets so much attention from young teens it is a bit weird why they still hardly ever have smalls in the west.
It must just boil down to the production numbers. Japan has 5 or 6 stores + web and the general population there is short and slim so they automatically get 98% of the smalls. It's like Supreme isn't taking into account to make more for the west or just too lazy/blazed to even notice.
That being said all the CDG stuff from this season was available in the US and UK in size small, along with PCL tee + hoodie, Crosses tee, Kingston tee and probably a couple more. Still, It's not a great deal when you consider how many other pieces dropped.