Well, I think you "can" eat as much as you want. But yes, in my house, a head of cabbage serves three. Sometimes four of my roommates boyfriend comes over. (I'm mean and make all of my friends eat veggies)
A whole medium head has 220 calories and a large head about 300, give or take. When you divide that by three people, its only about 70-100 calories each...or 1 WW point due to almost no fat and a ton of fiber. (On WW, one cup of cabbage is 0 points in general, but I always count it as one point since I'm sure I'm eating more than just a cup).
When I serve it with turkey sausage, which is 1WW point per ounce (normally we eat about 3 oz each) and red potatoes (3 points per 8 oz)...its a huge plate of food for about 7WW points. I don't do mini-meals so 7 points is pretty standard for dinner, maybe even on the low end.
In the grand scheme of things, I doubt eating too much cabbage once a week will keep me fat