This also depends on the DM. I would argue that if there is a backblast, the area of effect could be magnified if the concussive force of the blast is confined in some directions and not others. Therefore, if you detonate a fireball inside a narrow tunnel or hallway, both the area of effect and the backblast would increase considerably depending on how narrow the tunnel or hallway is. The range of the backblast can be longer than the maximum range of the spell, 150 feet. In gaming terms, the danger is that this can lead to a lot of fireball math, which can slow down your game tremendously. The solution is to make sure that there is a trust-based relationship between the DM and the players. The players should trust that the DM is not, pun intended, out to burn them. If the players have what most people consider reasonable backblast protection, then the DM would be expected to honor that without everyone having to break out paper, pencil, and calculator. Likewise, if one of the players happens to be a firefighter or ordnance expert in real life, and they say that their character has sufficient backblast protection from a 20 by 20-foot Fireball, the DM should trust that expert opinion and not assume that the player is lying just to cook a couple of extra orcs.