When I understand it right in German, "gleichwüchsig" refers to a group of trees, which all have the same height and this is also a valid meaning of "gleichwüchsig" when it refers to people.

It says "di meydl", which is surprising because it is singular "dos meydl" or plural "di meydlekh".

From my native German speaking feeling, I would say you are also right with "avarage build" and "symmetrical". I don't think the author of the above sentence meant obese when disrcibing the girl (girls?) with "gleykhvuksig" (Ger.: "gleichwüchsig")