Jaemin :
He would be so sad that you were feeling this way. He loves you with his whole heart and seeing you upset made his heart break a little. Of course he wouldn't ever show you how worried he truly was. Every time he'd see you after you told him that he'd make sure to tell you how beautiful you are even more than before.
Doyoung :
Hearing you tell him this he'd comfort you immediately telling you that he loves you no matter what. Obviously Doyoung wouldn't forget you telling him this and he'd do little things to help you be more confident like make some healthier meals when you two are together.
Johnny :
Johnny wouldn't care about your weight at all but how much you cared about it worried him. When you would tell him you were feeling insecure because of gaining some weight he would just hug you and tell you that he loves you no matter what.
Ten :
Immediately after you'd tell him you'd put on a little weight he'd be telling you not to worry about it. If you brought it up again it would start to really worry him. He'd start to make sure that you were taking care of yourself and make sure that it doesn't become something more. He'll always love you no matter what.