There are a couple of reasons for weight loss with MG.

The first and most obvious is the weakening of the jaw muscles and swallowing muscles make eating tiring - when you get tired eating you convince yourself you're full enough.

The other is that at least some of us who develop MG did so as a result of cancer - weightloss is frequently a symptom of cancer. If you have a thymoma this may be part of what is going on...