It seems like, for coldblooded animals you'll have an evolutionary advantage in size, as skin heat loss is lower the bigger you get.
If you have some heating from the metabolism, keeping heat in will can more important than absorbing it fast from outside and big size becomes a definite survival trait.

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‘To those who regard “crime fiction” as some sacred icon which must follow a rigid formula, I will always be the man who writes 18-syllable haiku.’Trying to make sense of computers, The Error Log.