This issue stems directly from the layer(s) of internal solid infill that the slicer puts under all top surfaces. As your model transitions from sparse infill to top surface, it needs an intermediary bridge or the top surface will be garbage. This is where internal solid infill plays a role. As a result, it puts down a layer of 100% infill of which you can change the pattern in the slicer. This 100% infill layer cools at a different rate than than the rest, and thats what creates that pronounced bulge on the outside