Well, I noticed there is an awkward incoherence between the size of buildings and their interiors, usally double the size of the external.

Is there a weird tax on the volume of buildings in videogames, causing bethesda devs to avoid making bigger buildings?

I mean this is quite ridiculous, so my question is quite sarcastic. Why the hell building interiors are double the size of exteriors? Is that an easter egg to magic tents in Harry Potter and the Goblet of Fire?

That's pretty common in games, actually. NWN did that due to the way combat and movement worked (interiors needed to be huge or you couldn't navigate or fight well). My guess would be that Bethesda just didn't want the players spending an over-long amount of time getting from one end of town to the other. Or maybe the smaller buildings make the towns feel more densely populated. I'm pretty sure the game eats a good portion of the 360's DVD capacity, too, particularly since they would have wanted to save space for the DLCs in a later version, so they may not have been able to scale the game up enough for 1:1 building exteriors.

Well, I find unclear how downscaling vectorial surfaces could have saved disk space, even if some textures can make use of it.

Well, it's not just scaling up the models, they'd have to fill in some other things to make the streets convincing, like rocks, lights, benches, etc. Larger cities would also either cut into the overworld land area or force them to increase the scale of the world to maintain the relative size.

Anyway, I really think it was probably more about making the cities less tedious to navigate.

Most likely is your last hypothesis. I haven't seen any games with such unpleasant incoherence, though.

Anyway, don't take me wrong. I paid attention to this detail only nowadays, after more that a thousand hour of gameplay, so this is not the worst thing ever; I really can't stop playing Skyrim, in spite of all its limitations, bugs and incoherences. I've finished Potema's chain quest, Meridia's dungeon and the Bard's college main quest tonight, with my Khajiit... 2-handed wielding mage. It was fun, as always. If my new pc is ever going to return home, I'm going to go on with my Nord warrior. With him I will almost surely join the Stormcloacks, in spite of the weird times we are living in real life.

This might be true. Having huge cities in Daggerfall was cool for a while but it did get tedious.