You might want to consider "they must like this food, it goes quick" as
something else.  If they're not getting the proper nutrients from a food
(and I'm not singling out any particular brand, I'm just making a general
comment), they will eat more of it to compensate.
 
One of the advantages of a high quality ferret food is that even though
each bag is more expensive, they eat less, poop less, and the poop smells
better -- still not like roses, but you can live with it.
 
Just my .02 worth.
 
Chana
[Posted in FML issue 3759]