Seems to just be based on passenger numbers, according to the boxes that appear if you hover on a station they are: Small: Less than 100,000 Medium: 100,000 t

Seems to just be based on passenger numbers, according to the boxes that appear if you hover on a station they are: Small: Less than 100,000 Medium: 100,000 to 5,000,000 Large: 5,000,000+ An unstaffed station local to me went from small to medium over the maps period and it hasn't had any major work done.