crys being able to change his body at will means he can keep track of his current weight, shapeshift like crazy, and then go back to his regular form with the default weight, so he could slowly gain over a long period of time just like anyone else if he wanted

it ALSO means... that he could gain with multiple different friends at once, keep track of his weights with each one. he can go back to those weights whenever hes around them, and then go back to his own default rate any other time

like hes all lean right. then his friend that hes mutual gaining with stops by, and hes like "lets see.. where were we..", and then balloons out to the size he was at with that friend. that sounds very fun