Shan,aside from the fact,IAMS and Eukanuba are not good quality food,I always used to think it was,they have an inhumane way of testing food on lab-animals(dogs/cats).
Although I never bought grocery-store food for my cats,I was never really aware of the differences until I joined this Forum.
I was told Royal Canin is a good food,so I bought it without checking ingredients and the second ingredient is corn-meal,the crows now eat it every day
Other good foods are Wellness,Chicken-soup-for-petlovers-soul(my cats love it) also Paul Newman has dog and catfood,good ingredients and most of the proceeds goes right back to animals in need.
In the long run,it will be cheaper to buy good quality food,they will eat less and not get fat on fillers.