While surface grows quadratically with scale, volume growth is cubic. Temperature exchange (gain and loss) of animals is mostly dependant on surface area (the more area, the faster) whereas heat generation is largely dependant on mass / volume. For warm blooded animals, this implies that specimen in colder climates are usually larger than members of the same species in warmer climates, in order to lose less heat relative to their heat generation.
I'd expect cold blooded animals to approach the environmental temperature faster if they were smaller, having a more suitable surface-to-volume ratio. This obviously is of advantage when warming up, but should be as much of a disadvantage when cooling down. So how does surface-to-volume ratio matter for cold blooded species?