When outside the bus the average distance to various points of the bus must be at least twice as far away as the average distance when inside. The discrepancy of apparent size is a perceptual illusion due to distance and convergence and divergence of linier perspective.
Compare convergence and divergence of lines between an inside corner and an outside corner from the same distance. The illusion is apparent even in the two dimensional representation of a photograph.
However if you are not satisfied with this answer (nor am I with my ability to explain it) may I suggest a fancier explanation . . .
. . . look into
Oculomotor Micropsia/Macropsia