This is what I know of Predisone. It is a steroid.
Now, I hope you don't get offended about my example.
Here it goes. . .
My mother and dog were on the same dosage. My mother did not gain weight and had been on the drug for years. My dog was on the drug for about two months and got super-sized overnight; I guess because his body depended more on the drug. His body did give out on him, but after all was said and done he fully recovered.
I know you are freaked out by the weight gain. Perhaps, you can talk to your doctor about another option, but if it is working for you. . .
You might want to reconsider.