I often see better than a 15C difference on sunny days in my greenhouses unless I vent heavily. Anything above about 37C tends to be pretty hard on the plants in the humid, closed greenhouse environment, so my ventilation gets more aggressive in hotter weather.
Glass is a very poor insulator, with high loss rates in both radiant and contact heat. Iron frames also tend to leach away heat. I don't know that I would count on the ground being your biggest source of heat loss, though it will be a significant heat sink, retaining both heat and cold.
