Blue Maxi dress size M, fits like a small

Blue, Navy, White patterned maxi dress. Tag says size M, 5/48, but I’m 5’4 and usually wears small, and it fits me perfectly. Purchased at Off Saks 5th Ave, wore once to a wedding rehearsal. Super comfortable and flattering jersey material. Measurements: 49 inches long, 13 inches across, but very stretchy.