If one thing is incommensurate with another, it doesn't fit or is out of proportion. If your expensive meal doesn't taste very good, you could say its quality is incommensurate with its cost.
Things that are incommensurate just don't match — they're disproportionate or off-kilter. Your little brother's loud mouth might be incommensurate with his tiny size, and your small appetite could be incommensurate with the enormous banana split in front of you. The commensurate part of this word comes from the Late Latin commensuratus, literally "measured with," or "equal," and the prefix in- means "not."