I have all of the sizes (except the XXL), and the small is the only one with heat issues. Its an air flow problem. With the smallest air inlet to volume ratio, it has the highest chance of having issues. The other issue mine has (and I think some, but not all small owners suffer from) is that the firebox and fire ring are more narrow than my other eggs. It doesn't fill the cavity, leaving a larger gap for air to go up around the firebox. My other eggs only have one path for the air, through a lump burning fire. I have mitigated this by creating a foil rope around the fire box, and that made my small act more like an egg should
I have since left my small to lower heat duties, and it is fantastic at that. The medium does nuclear duty now. The medium has a large inlet, and a smaller volume compared to the large. It is a fire breathing monster.