You might find if you are working them fat, then they will show less of an intrest in food! (I.e live rabbit).
Out of season don't worry about a fat ferret, it will do it no harm.
Think the biscuit stuff packs the weight on. (look at the Atkins diet, no carbs. What's in the biscuit?)
The odd day missed from feeding will do no harm, i'm sure in the wild they will miss the odd day from time to time.
I split mine into cages to control the weight, and put like for like animals together, some are big eaters, and others lose condition when caged with these animals, due to the food hogging, and s