It makes him look fatter? This isn't right. Henry was quite large already so didn't need to look bigger.
It wasn't just a style at the time either, but was done to have an effect.
The correct answer is that it was done to make him look elegant. By making someone's legs look longer it makes them look taller. A fat body then looks in proportion to the legs and the figure looks more elegant. Holbein was flattering Henry.