"Owlina eldest children are expected to be a good example to their younger siblings."
This is terrible. It's really awful to assign roles to your children like this and then reinforce them with material rewards and disadvantages.
The way you treat children affects them for the rest of their life. the fact that they have a temporary condition of being young, younger, older, middle, whatever, within your family as children, should NOT become a template for their imagined "personality" that they can never escape till they die.
Yes there is a tendency for parents to unconsciously expect more of the oldest (who is behaving like a 6 year old when the little one is behaving like a 4 year old - but they won't be 6 for ever! the other one won't be 4 for ever!) but you don't just give in to this and force your poor pfb into the one that always gets responsibility! this idea that "if I don't manage it it won't be done" "I am the only one who can control anything" "if it goes wrong it's down to me" is just awful. and you enhance it by adding "after all, I have the status and trappings of a natural leader so I have no way of stepping down even for a moment"
Similarly you don't always expect the little one to be silly and irresponsible, because this is an entirely inappropriate self image for a person going out to secondary school, to university, or to work, or forming relationships - and it will stick. This person internalises that what they do or do not do is trivial and doesn't matter, no one cares about their opinion, their boundaries are irrelevant, they are not worthy of respect, and working for anything will be ultimate fruitless as there are others who just intrinsically deserve status nd power, not them.
honestly these things do stick.
there is not one single good argument on this thread for why it is correct to assume that the big room goes to the eldest, and any attempts to explain it are just examples of awful, damaging, stereotyping behaviour.
Please think about your children as people! not as units on your mental diagram of the structure of your family!