A variety of things. More space is definitely harder to heat evenly, especially without airflow coming from something like a convection oven, or keeping relativ

A variety of things. More space is definitely harder to heat evenly, especially without airflow coming from something like a convection oven, or keeping relative humidity very high, such as in a combi/steam oven, but a lot of it is cost pressure. If you go far enough up market to Miele, Wolf, etc., you can find ovens that will keep a much more consistent temperature.