Cat Food Change

I switched to a new cat food brand for my cats around a week ago. As kids, they were fed with Science Diet or Iams. After they got to HK, I could only afford Whiskas because Whiskas in HK was about the same price as Science Diet in Toronto. Since Mew had her teeth problem, I thought my senior cats deserved some better food. I bought them a bag of Iams Hairball Control. After the switch, I noticed immediately the change in their poo. Their poo is now firmer and bigger. It also doesn't smell. Mew's diarrhea has stopped instantly. Now I don't even have to worry about having to take her to the vet anymore because her diarrhea has been going on for a month. My cats also don't seem to be as hungry as easily. I guess there is a reason for the price difference and the reputation of the good brands.