Total model size

You cannot add Premium and PPU licenses to get more capacity. 

P1 has a memory limit of 25GB, but that is a relatively small model size. MS recommends the PBIX be 3GB or less for a P1. That is because it is compressed and can greatly expand into RAM depending on how many users are accessing it.
PPU has model limit of 100GB per model, which is vastly superior to a P1 or P2, but it requires every user have a PPU license to access. But once they have that, you can have dozens of 100GB models one or dozens of PPU workspaces. Very very powerful. But may not be cost effective for large organizations. P2 and P3 (and larger) capacities generally are better in those cases for a cost per user metric.