Taller people tend to have larger hearts and wider artery's which makes a difference. It's pressure = flow rate / diameter of pipe ^ 2. So, as you gain weight your arteries can get wider or the pressure can increase.

Gaining weight increases skin, which is why skin reduction surgery happens for people that lose lot's of weight. GIS for skin folds.

Weight / height ^2 is a proxy for both it's far from perfect, but it's also really easy to collect.