It is a delicate problem but due to his anatomical shortcomings,  Bart's stereoscopic vision is a little underpowered. Readers may have noticed that from time to time Farmer Brassica is a bit out in his estimation of how big things, or in this case, how small things, really are.  This is a state which Mavis Eggwhistle has tsked tsked over more than once.
Bart purchased this barn from a garage sale in Flemington,  thinking it was some ways off , when in fact a moment later he banged his funny bone on the roof.  Bart has decided the barn will make cheerful accommodation for farm animals, and maintains he knew all along it didn't come past the buckle on his overalls.

PS If you're wondering about that fancy wall  behind the barn , it  is Bart's prize winning tromp l'eol.