We call this style the “big butt” as it’s from the period just prior to the flapper era when everything started getting all angular and deco (you can’t look like your gramma, can you?).
Notice the little piece of light-colored material between the heel and the heel cap (at the very bottom). Very popular during this era.
It looks as though the “butt” is exaggerated due to the leather on the throat of the boot shrinking. That may be… a little… as canvas shoes of the era also have a big ol’ butt – and they did not shrink from being left in a hot attic!