Because taller players have an obvious height advantage. A 5'7" player has to jump 8 inches for his head to be at the same height as 6'3" player so there is an obvious significant advantage. When it comes to strong compared to slim (I assume you mean big built compared to slim as strong and slim makes no sense, its like comparing the number 6 with fish and chips) then the bigger players have more weight to put into their challenges. They can lean on the other player with a bit of weight and that is obviously a big help too. Of course not all big tall players are better in the air than short slim players but physically they have obvious advantages.