Why are killer whales found in the Gulf of Mexico smaller than ones found in the Pacific? I recently found out that there are killer whales that live in the Gulf of Mexico, but they are smaller than killer whales that are commonly found in the Pacific Ocean. Why is it they are smaller in size?

Animals living in colder climate / environment are larger because this way they have better body surface / volume (mass) ration thus they're more resistant to cold. Animals living in warmer areas are smaller thus they have larger body surface compared to their volume(mass) to dissipate heat easier. Check this wiki page for more details.