A bigger disk means you can store more "plots", which are the the files necessary for proof-of-storage, and thus makes it more likely that you will earn a block

A bigger disk means you can store more "plots", which are the the files necessary for proof-of-storage, and thus makes it more likely that you will earn a block reward.. But in order to first generate these plots, you have to "farm" them which is a very write intensive process which is ultimately what is at focus here. The creation of these files requires so much writing that you are basically writing as fast as your CPU can, until the drive dies.