Just from the shape of the boiler? Huh.
It is probably a heating surface area vs area exposed to the atmosphere thing, along with surface tension (related to the atmosphere “window” size). I read something about a long time ago.
I’ll see if I can find a reference. I probably have a water boiling point folder someplace, on a drive. Apparently the experiments with “superheating” water can be quite dangerous. Spontaneous giant vapour bubbles (can throw all the other hot water out of, or break the container) and instant vaporization of all the water at once are potential hazards.