“The more the merrier” or “the sooner the better” or “the bigger they are the harder they fall” can you see a similarity between all of these idioms?
We use the + comparative/the + comparative to show one thing depends on something else: In the case of “The bigger they are, the harder they fall,” how hard the person falls depends on how big they are. The idea here is that if someone has a lot of money, fame, power, strength, etc. it will more difficult for them to lose those things (fall).