In Pokemon diamond what is the fat guy in victory road blocking doorway for?

The "fat guy" you're referring to in Victory Road in Pokémon Diamond is actually a trainer named Riley. He is a member of the Galactic team, and he is guarding the entrance to the final portion of Victory Road to prevent anyone from reaching the top and challenging the Elite Four.

Riley is a powerful trainer, and he is part of the Galactic team's plan to take over the region. He acts as a roadblock to prevent anyone from stopping them from achieving their goals.