fuagf -- too warm -- mammals generate their own heat and (leaving aside colder, well below body temp, environments) need to shed heat; so when it gets warmer, they get smaller to be able to continue to maintain a body temp that isn't too warm -- on the other hand, reptiles rely on drawing heat from their environment, so warmer environment and they can get bigger and still be able to stay warm enough to do what they gotta do
it's a surface area (and corresponding capacity to shed or absorb heat) v. volume/mass thing -- the ratio of a thing's surface area to its volume/mass decreases as the thing gets bigger
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