It may be less about fat content and more about the bio-availability of the nutrients in the food. His body is using more of what he is eating, instead of pooping half of the food back out.
However looking at the link, those do seem pretty high in fat. Compared with the Rad Cat I feed:
http://www.radfood.com/site/856/LabAnalysis2012.pdfA question though, is he gaining weight, or getting fatter? With increased exercise, and better diet, he will build muscle, which weighs more than fat. So if you are basing your observation on weighing him, maybe he's just gaining muscle. If he looks fatter, maybe his fur is just thicker from the improved diet. If he FEELS fatter, well....that's different.
Maybe add a leaner brand into the rotation? Or cut back a little on quantity. Sometimes as little as .25 oz can make the difference.
I've had to cut back. My cats who were eating 4.5 ounces a day now only get between 4 and 4.25 a day (combo commercial raw, PMR and canned) because of weight gain.