The higher quality food has less fillers, so it's normal for the cat to eat a bit less of it. In other words, the cat has to eat less in order to get the energy and nutrients it needs.
Try mixing the good and bad foods together for a week, each day increasing the proportion of the high quality food.
EDIT: He's a big boy. I would feed the high quality. You can think about supplementing with dry, but that depends on your preferences and the cat's health. Any cat with kidney or urinary tract troubles should stay on canned, unless a vet says otherwise.
I honestly wouldn't worry too much. The worst thing that happens is the cat loses weight. Unless he has a very large build, it sounds like losing a little weight might even help him.
Once you're switched over, watch the cat's weight for a month or so. If he drops enough weight that he's underweight, then consider changing him back or adding in some low quality food. If he just drops some weight, but is still healthy weight, just keep him on the food.
My cats don't eat nearly as much as their weight suggests, but they're both healthy and happy.
Finally, you can try warming the food a bit in the microwave. Some cats don't like cold or even room temperature wet food.