Is there a lore reason why so many bosses in dark souls that are basically just people in armor are fucking huge...

They're just superhuman. Probably some kind of powerful pre-human hominids that eventually degenerated into normal people over time.

You're actually a manlet

I always saw the Dark Souls world divided into Gods and Humans. Humans being smaller compared to the Gods who were just beings that were really tall.

Wisdom

Some demi-god race. Both Gough and Ciaran say something along the lines of 'typical human treachery/behavior' when you antagonize/kill them. Lazy answer God magic that makes them big.

it's because they eat their spinach

The split is something like "the gods" being the ones in charge of everything, "demigods" for lack of a better term being those who work directly for the gods, giants, and humans. Granted, there's almost no information outside of item descriptions and dialog that is vague if you aren't throwing everything at the wall to see what sticks. Since humans are those that come from the Furtive Pygmy and carry fragments of the Dark Soul, that's the reason we're so small compared to everything else.

This seems like the most reasonable answer.

I once made a thread related to this question but from a different angle. My question was whether there are any games where we get to be the the giant. So far the closest thing to this would be games with character creators where you can create a fuckhuge person as your avatar and then constantly fight against bandits who are usually average sized.

In Souls' case it's perhaps because humans are descended from the Pigmy guy who by definition would be smaller in stature than the other gods, but the games don't really explicitly say that.

This is why I always saw the Dark Lord end in DS1 as the proper good end since it meant the Age of Humans was finally taking place rather than the endless loop with the Age of Gods