You're bringing this as if there is an objective measure for the 'needs' of the living space of two people, and that what is considered 'normal' is way above that objective measure. You're not bringing any supportive evidence for this (in my eyes) grandiose claim, other than proof by vigorous hand waving and stating it in terms of great certainty.
The one objective measure one can bring to the table is the lower boundary - which is maybe around 2 m2 per person, as there are numerous houses (e.g. dwelling where illegal immigrants are housed in by human traffickers) (I'm just making a guess here, based on hearing for example about 50 people being packed in small houses in farming areas). I suppose you're not seriously arguing that this is what we should strive for, yet this is (objectively) how little room humans can 'survive' on. (if it were possible on less, these people would get less, considering human traffickers probably try to minimize their costs in order to maximize profits).
You quantify your statements by saying that 30m2 is 'livable' for two people. I don't see how you can make this assertion. What is 'livable' or 'comfortable' is entirely subjective. One may get /used/ to living in a smaller room; heck, one would get 'used' to being sodomized daily with a coke bottle too, I guess, as long as it's done for a long enough time. That still doesn't mean it's something to aim for. You're projecting an entirely subjective /opinion/ (houses should be small, people should not have much stuff which is implied by living in such a small house, ...) onto this discussion and present it as /fact/.
Anyway I'm perfectly willing to accept that there is an 'optimum' in the sense that the median of the population would feel comfortable enough living in an apartment of a certain size. I know of no way to measure this optimum, because so many of the variable are hard to quantify. Instead we could say that the reality shows preferences already: the amount of space people feel most comfortable in, in function of their spending capability, is the house they live in! This may not be perfect, but it sure would show that what 'people' in general find 'comfortable enough' is a lot higher than your 30 m2.