Something that's ill-proportioned is oddly shaped or sized, or doesn't fit in relation to things around it. Your tiny oil painting is going to look ill-proportioned in the middle of that huge, white wall.
A brand new, massive, glass-covered skyscraper might look ill-proportioned in your city of mostly historic brick buildings, and you might find that your big, clompy rain books are ill-proportioned compared to the rest of your outfit. This adjective adds the prefix ill-, "not well" or "badly," to proportioned, which comes from the Latin root word proportionem, "comparative relation."