One could say that Linden Labs screwed up twice.
Once was with how the official viewer measures the height of an avatar. Not to the top of the head, but to the eyes which means that it's inaccurate by about 6" or 15cm. Since it's inaccurate, users have stopped paying attention to the avatar height measurement altogether and started adjusting their avatars' height until it looked roughly okay.
And then there was the default camera angle. Classic 3-D third-person "platformer" angle, namely behind and way above the head. Every last viewer has taken this over.
For one, this means trouble in all buildings with ceiling heights under about 12' or 3.60m. If you're lucky, the camera will constantly collide with the ceiling. If you're unlucky, the camera will end up upstairs.
Besides, builders in both Second Life and OpenSim tend to take whatever height their camera is at for eye height. This explains some sims with ridonkulously oversized buildings because whoever built them has been flying all the time. But it also explains why houses are even too big for 9' avatars: Whoever built them built them as if the eyes of their avatar were as high as the default camera.
Both contributes to how oversized almost all buildings in OpenSim are. If your avatar is realistic, door handles may be at eye height, railings around porches are as high as your chin, and some windows are so high up the walls that the avatar needs a ladder to look outside. Even if the building has a kitchen built in, it's sometimes even too big for a 9' avatar because it had to be adapted to the overall proportions of the building in which the kitchen could double as a boat shed.
Sim builders like Xi Shi or Suzi Avonside build the houses on their sims at realistic sizes, but they constantly have to shrug off comments by users with oversized avatars that their buildings are too small.
The Firestorm Viewer has taken a first step at fixing the camera height issue. Version 6.6.8 has introduced Penny Patton's camera angle (see
https://pennycow.blogspot.com/2016/11/the-improved-sl-came... and
https://modemworld.me/tutorials__trashed/camera-offsets-pe...) with a few modifications as the new default camera preset. It places the camera behind the head with only minimal height offset.
All of a sudden, you can navigate the in-scale buildings in Ruritania or Paradwys without your camera bumping into the ceiling. And all of a sudden, you feel like a dwarf or a small child in many typical Second Life/OpenSim houses.
However, it's still only this one viewer in this one version which is buggy enough for many not to upgrade. A few older Firestorm versions have camera presets, but you have to enter the Penny Patton values yourself. All the other viewers don't have custom camera presets, and you have to jump through hoops to edit the camera position, if it's possible at all.
By the way, a side-effect of the default way-up-high camera are extremely long-legged avatars. From that top-down perspective, your legs look shorter in comparison with the rest of your avatar than they are. Since people aren't used to looking at other people from that angle, they find it looking weird. And they go and increase the length of their avatars' legs until they look okay from the default Linden camera angle.