Is it possible for S sized creature to hit a creature mounted on a L sized creature?

@danielceagleIs it possible for S sized creature to hit a creature mounted on a L sized creature with melee attack? DM's call in the end, but I'd say yes – assume legs hang down, etc. #WOTCstaff

— (((Mike Mearls))) (@mikemearls) July 30, 2016

Is it possible for S sized creature to hit a creature mounted on a L sized creature with melee attack?