While Goretzka looks like he packed on pure muscle, Coutinho looks (especially in the face) like he put on some muscle and some body fat. Nonetheless, he still moved very well and performed on the pitch in the games he came on. I doubt he put on more than 5-10lbs or 2-5kg and for a small guy with low center of gravity who is already explosive and has amazing command of the ball and his body, it is not much. I do wonder had he not come on as a substitute and played the full match how well he would have lasted since carrying around more weight is less efficient.