I tried to answer this with "he is not big. he is small" and it was marked incorrect. should this not be a valid answer as well?
I suppose so, though I think people would be more likely described as woch "tall" and run "short" rather than as tIn "big" and mach "little".
People can be big and small in ways other than just height.
Indeed but even without it making sense, i was more concerned with the grammar. Thanks for clarifying!