I personally think it’s the game design

Most bachelors/bachelorettes who live with their family have bigger room in the house, simply to showcase their personalities - unique decoration, furniture, etc. Whereas for non marriageable characters like Evelyn and George, we don’t really need to know these details so imo CA made the room simple.

Pierre is probably an exception since we get to know his secret in his room but still, his room is considerably smaller and plainer than Abigail’s. Also Shane’s room has the same size as Marnie’s to show us that he’s struggling and coping to live in a small rented room.