You will lose body mass, yes, when you lose fat, because you are losing fat mass, but the only size you will lose if you lose FAT is the size caused by the fat. Muscle mass does NOT have to be lost, and the PRIMARY aim of cutting should be to MAINTAIN YOUR CURRENT MUSCLE MASS.
Everyone does things differently, and the Pros don't always do things the most sensibly, either. In fact, you'll notice that a lot of them do not stay in a perpetual caloric deficit when cutting for a show, but calorie/carbohydrate cycle, which is the best nutrition/diet method for effective and successful fat loss.
On another note, cutting carbohydrates and upping protein does not mean that one will maintain their muscle mass and only lose fat if they stay in a caloric DEFICIT. It also depend on the deficit and how long one is on one, as well as a host of other factors when it comes to what ratio of fat:muscle/lean mass is lost.