And you forgot that he has his original automail arm replaced with a much lighter one during his time in Fort Briggs, which impeded his growth rate due to how heavy it is. Later on, in one episode where it shows him and some of his new companions (being the two chimera he fought on his own and Greed/ Ling) back in the Rockbells' home, there is a scene that shows the height difference of Winry and Ed with his new lighter automail arm.