Done a lot of thinking about this, Hydrocortisone is the natural replacement for cortisol, so if given at the right time and the right dose, then you should not gain weight. Look how those who dose in the circadian rhythm have had such success, this  new pump is also set to deliver the hydro to follow the body’s natural circadian rhythm. Essentially dex because it is long lasting, peaks and then tapers down throughout the day - so it sort of follows the circadian rhythm however it has terrible side effects if given in the wrong dose, as it is so much more potent and does not suit all. If you are gaining weight on hydrocortisone, then I would think it is probably because the doses given maybe too high and need to be tapered to fit the individual.  Some people do really well on 2 doses a day, where as some people need the same dose but in smaller doses throughout the day.  I find it very surprising that you would lose weight on dex which is stronger,and gain weight on hydro which is the natural replacement - hence my theory (which is again is my thoughts) smaller doses of hydro at the appropriate times, and you would not gain weight.