Unless the Cushing's Syndrome somehow implies termodynamics laws' violation, which I strongly doubt, I don't think you can increase *weight* simply by metabolizing tissues into fat.
Yeah, OK, you're right about that--especially your point about water content of fat. It's been a long time since I dealt with this with a relative, and my recollection is somewhat hazy of the explanation for why it can be simply impossible for Cushing's patients to avoid weight gain, much less lose weight.
And yes, I know Cushing's Syndrome usually curses with obesity, but you can bet it is not because of what you say but because the way you eat more (specially when it curses with depression) and how you transform what you eat mainly into fat at the expense of other tissues, which in turn makes you move less, which in turn makes your intake even more -relatively speaking, excessive.
Cushing's patients can gain weight even on strict calorie-limited diets and with vigorous exercise. It's because of the changes in the way food is metabolized, increased metabolism specifically of carbohydrates & fats. In order to actually lose weight, some people with that disease would have to reduce caloric intake to a level that would lead to death pretty quickly from heart failure. (Now, to be clear, I am not talking about the mildest cases, thus "can gain weight..." not "will gain weight...", "some people..." not "all people...".)
Let me put it to you this way: when said relative described her attempts at calorie reduction and increased exercise to the specialist, his response was "Yes, that's why you gained 100 lbs last year instead of 200".
The vast majority of overweight people simply need to eat less and exercise more. That's true, but unfortunately it has been simplified into such dogma that, literally, someone who has weighed 120lbs an entire adult life, and suddenly balloons to nearly 200 in a few months without diet or activity changes, can go to the doctor and be told to see a dietician and and get a personal trainer, despite having multiple other symptoms besides weight gain, and that's dangerous. Dangerous, and happens again and again--very common at the Mayo Clinic for Cushing's patients to show up with photos to prove what they looked like a year ago because they've been blown off by so many.
Average time from onset of symptoms to diagnosis: 3 years. In a disease that has a 50% mortality rate (mostly heart failure) in 5 years. While 3 < 5, that's a delay that is scary :-(