It’s not a name for any disorder, if you ask about it. I believe it just means the same as English ‘small head’, a head that is small, by any measure used by the speaker. One could use it to describe microcephaly’s symptoms as well as to describe person with a somewhat-small-but-healthy head. Or maybe metaphorically to describe a not so clever person (as opposed to a genius with metaphorically giant head).
If you want your translation to be added, then you must provide evidence that confirms your suggestion. We can't just add stuff based on hearsay.
The only expansion of 'mam głowę' that I'd accept is 'mam głowę na karku' or 'mam głowę nie od parady', neither of which has anything to do with headaches.