My theory is the cookie could be smaller because Ellen's cookie might have been smaller as a full than Joseph's.
For example, Joseph's cookie may have 24 pieces, while Ellen's cookie may have been 4 pieces.
The Cookie that joseph was eating was larger. Half of a small cookie is smaller than a quarter of a larger cookie.
Joseph's quarter of a cookie can be larger than Ellen's half of a cookie if the whole cookie that Joseph was taking from is much larger than the one Ellen had. For example, if Joseph's whole cookie weighed 200 grams, then his quarter (50 grams) would be larger than Ellen's half of a smaller cookie weighing 80 grams (40 grams). Thus, fractions alone do not determine size; the total size of the original cookies matters.
;