Most of these Eastern Europeans hit hard

It's definitely the way they train and are bought up, the Brits, yanks are more Canadians, South Americans don't train strength and power like the old Soviet Union countries do. The Athletes of the western train speed, dyanasim, skills. Football, american football, basketball, rugby, swimming, athletics. Not to say the Eastern European s haven't produced great athletes in those fields, but the old soviet states thrive in weight lifting, powerlifting and strong man competitions. So I think it's genetically imprinted in them that they are very strong, robust fighters. The power isn't something they needing teaching, and that's why are seeing such beasts from the eastside dominating the middleweight to heavyweight divisions at the moment. Remember boxing has nothing to do with who is the strongest who can lift the most, far from it, it's often a handicap to have too much slow twitch muscles, but the ones who learn how to box and don't rely on the strength are doing well. Tl:dr these Eastern Europeans are very strong either naturally or through nurture.