I've never really been able to get my head around this issue, but as I understand it, ME per KG is a more meaningful number than KCal per cup. I'm just not sure why though since I feed by the cup
3 foods. #1 is 3,550 ME and 410 kcal per cup, #2 is 3390 ME and 346 per cup, and #3 is 3400 ME and 298 per cup.
My small dog will gain weight on #1 and be a bit heavy even with regular exercise. She will maintain on #2 but will lose weight and tone up if exercised regularly. And on #3 she maintains a lighter weight and good tone without exercise. So from my standpoint, I see a wider discrepancy between these three foods based on their published Kcal per cup. There doesn't seem to be a lot of difference in the ME.
And I've seen other foods with a ME of around 3400 and kcals at anywhere from 315, to 340 to 360. I'm assuming that the density and size of the kibble can make a difference and the digestibility is also a big part of the equation.