The grandiosity is an odd thing to contemplate! I don’t see it in all of the autistic people in my family at all.
My dad doesn’t boast at all. Ever. But I think he simply doesn’t struggle with low self esteem. He doesn’t question himself, if people don’t like him it doesn’t bother him. Therefore he seems pretty comfortable with who he is. So no boasting or grandiosity required. He does criticise people all the time. But he also compliments people (to excess!) if he thinks they’ve done something well. Perhaps he feels that his judgement is important! Judging us all, passing his comments. A superiority of sorts! But I don’t think he thinks he is better than anyone. Apart from people who vote Conservative. He definitely thinks he is better than them!! (And there we agree!!!)
My SIL does suffer considerably with thinking she is being criticised and is extremely defensive at all times. Hers is a more subtle form of boasting. It’s not the puffing out the chest kind. She is seeking validation. And is always convinced her way of doing things is the right way, and she needs to have it constantly reaffirmed how very well she is doing and how right she is. Less “look at meeee!!!! I’m the best!!!!” And more “I did this because I am right, aren’t I? See how my way is best? See that I was right? Aren’t I? See how what a good parent/wife/friend/employee I am!!” I think this comes from self doubt and a need to big oneself up in order to get validation.
My DSS boasts rather like a small child. Although he is teen. The puffing out the chest and proclaiming how much better he is than others at things. But he has low self esteem also. I think this may have come about because my DP had it in mind that his son was some kind of genius and went about boasting to everyone how clever he was, even though he was only 2! DSS was raised to think he was incredible! And of course when he went to school and couldn’t keep up with the other children who in Y1 were reading and writing and adding up….it was a long way to fall! And it damaged his self esteem irreparably. So he boasts because I think he feels like he isn’t good at anything. And he also seems not to have a single nice thing to say about anyone. It’s constant criticism all the time of everyone. Sneering comments about how rubbish everyone is. I can’t watch TV in the same room as him anymore as it’s just a barrage of sneering at everyone on the screen!
DP also boasts like a child. I’ve always found this strange about him. How important he is at work, how attractive he is, how many friends he has, how much money he spent on a thing….as I say, very childlike boasting. Cringey. Although he lacks the social compass to recognise this. Unlike the others I cannot fathom whether this comes from low self esteem or not. Seemingly it must do. Why else would someone feel the need to go on about these things to other people?! I think he must suffer with self doubt (though he hides this extremely well!!) and I think this is him trying to convince everyone how amazing he is. I also think he must lurch between genuinely believing he is amazing and feeling the need to convince others of his amazingness. This is why I think so many people think he’s a narc. He genuinely comes across, not as a vulnerable person seeking validation, but as extremely arrogant. The superiority is high with this one! He, like his son, feels the need to criticise and criticise and criticise. And I have always struggled as to where this comes from. I feel like it must be low self esteem, but it really doesn’t come across like that!
TLDR: the grandiosity and superiority is a very complex issue. It’s not universal and I think comes from very different places depending on the individual.