This has been bothering me recently. Look at any old school map, like the example below:
You would think that each of the halls here are 10' wide if you assume each square=10'. But unless those walls are extremely thin, the hallways are probably closer to 8' wide. That's not to say that the 10' wide hallway doesn't exist; any situation where there isn't another hallway or room on the other side of the wall would allow for them.
Does this affect gameplay? Should I use these new hallway sizes when calculating the volume taken up by my fireball spell? No. It's just a pet peeve of mine.