(Though, I will say, the URIs returned for Twitter and the image sizes don't actually align in those cases. The first is actually 192ˣ192 pixels, and the second is 32ˣ32 pixels. That seems odd. Maybe they should have endpoints like domain/large, domain/medium, domain/small?)
The number after the domain is meant to be "this or the next greater size"; the actual size depends on what the source web site offers.