Anyones baby on enfamil nutramigen formula?

my son is four months old and on the enfamil nutramigen formula. he is on the concentrate, but hes going through what would be the euivalentof 10 acans of powder a month without any waste. but hes not gaining weight. like my other two babies did. by this time they were BEYOND chunky, but he still long and skinny. all o fmy kisd have been long, but the other two were porky too lol. i know every baby is different, but since the nutramigen is processed differently, i was wondering if that mya be why he isnt gaining weight like the other two did. howver the drs are not concerned. hes still within the weight range he shuold be, just not as chunky.