Eats less formula?

My son is 5 weeks old, whenever I give him breastmilk he drinks 4 ounces every feeding (every 2 hours ) I have started to give him formula two times a day so I can up my breastmilk supply for when I return to work soon(pumping and storing in freezer).

When I give him the formula he is only drinking 2 ounces. Most of the time after drinking the 2 oz he falls asleep immediately to where I can't even get him to burp sometimes. My question is, is it OK that he is only drinking 2 oz of formula for some of his meals? Why would he drink 4 oz of breastmilk but only 2 of formula, I know formulas more filling but shouldn't he still drink more than that?