I only feed a small amount as they don't need to gain weight but I like the gloss it gives the coat though my ID couldn't really have much more as it makes her poo quite wet.
I'd start on 1 mug and see how you get on, I am now feeding 3 mugs to mine and it doesn't put the weight on them, they are natives too. I'm hoping it's just maintaining their condition and they have some room to fatten up this spring. Lovely shiny coats on them
I feed 2 mugs a day now to semi retired native, in proper work he used to get two in summer and 4 in winter but he hasn't needed the extra this winter. He didn't put on weight on the 2 a day in summer while dossing either though .