If a boot isn't excruciatingly-painful when you first put your foot in it, out of the box, then it is too big. And if somebody is compensating for too much longitudinal length with a narrower last, that doesn't make any sense either. And, if there is a bunch of room behind your heel in a low-volume shell, you've got the wrong shell.
Well some people have a different description of the same pain. Excruciating pain can be holding you foot in the fire until there's nothing left of it to one person, but it can be a tight fitting boot to another.