Really different portion size for different brands?

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Purred: Sun Feb 3, '13 4:23pm PST 
I've been feeding Furry Foodie which says feed 6 oz daily for every 8 lbs of body weight.

My cat is about 8 1/2 lbs, so I've been giving her 3 meals of roughly 2 oz each.

However, she's been having some tummy troubles, so I'm trying a different brand... Instinct Raw Frozen Duck. According to their calculator (http://www.naturesvariety.com/feedguide), a 9 lb very active cat should eat 3/4 cup of Instinct Frozen Raw Bites Duck a day.

However, I didn't read the instructions before measuring out her meals, so I just weighed out 2 oz portions. It looked like about twice as much as her furry foodie meals, and indeed she only ate about half of it.

The 2 oz of Instinct Raw Frozen Duck looked like about 1/2 cup (just eyeballing it) when the recommended portion is only 1/4 cup. Since I was using a food scale, my weights are precise, but my 1/2 cup vs 1/4 cup is a rough guess.

So, at the end of all of this, I'm wondering why my cat should eat about 6 oz of one brand of frozen raw cat food, and what seems like half of that of another brand of frozen raw food.

Using the frankenprey model, a 8.5 lb very active cat should require just under 6 oz of food a day.

So what gives?