Ross has it almost right...
Even a half full tank can spill into the vent line when the boat is heeled if the vent fitting is outboard on the tank...doesn't matter what the diameter of vent line is. If there's a sag in the vent line, waste will be left sitting in it. But a "wide spot in the road" that will allow air to bubble through any waste sitting in the line is not the answer, 'cuz waste allowed to sit in the vent line will eventually block it. Even if it doesn't sit in the line, it'll still build up and block the vent at the tank and at the vent thru-hull. So the solution to the problem is: straighten out the vent line...and to makesure the vent doesn't ever become blocked, backflush it every time the tank is pumped out and/or you wash the boat. If the vent thru-hull isn't a design that will allow you to do that, replace it with a thru-hull that will.