All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
According to the Sharee’ah, a wife is obliged to obey her husband in what is permissible. In fact, the scholars stated that she is obliged to obey him in matters pertaining to marriage and enjoyment provided that this is permissible and provided this does not cause any harm to the wife. You may refer to Fataawa 91208 and 91788 in this regard.
A husband asking his wife to gain more weight may pertain to the matter of enjoyment, in which case, she is obliged to obey him in this matter provided that this does not cause harm to her, otherwise she is not obliged to obey him in this case.
With regard to your statement “I cannot be 10 kilos more than I am now”, if you mean that this may not happen even if you take the proper means for that, then this is beyond your ability and Allaah Says (what means): {Allaah does not charge a soul except [with that within] its capacity.}[Quran 2:286] Therefore, in this case, you are not sinful if you take the permissible means and what he wishes does not happen. But if you mean that gaining weight is a matter that is difficult to accept, then this is improper because it is contradictory to the obedience of the husband in what is permissible.
In our view, the matter is easy and it is a matter on which the spouses can have a mutual agreement and this does not need to cause tension or any kind of problems between them.
Allaah Knows best.