I confess to knowing the answer to this because I look around 
It seems that unless you order a la carte a standard breakfast will be something like two to three eggs, three or more strips of bacon, potatoes and toast. Bob and I always share breakfast and it’s plenty, sometimes MORE than plenty. I would never eat that much at home. Would you? I’ve started having little convos with others about this and they don’t know why they don’t share. And servers are telling me that rarely does anyone finish all that food. Let’s first of course eliminate single diners cause they have no choice. Recently an old friend came to visit. Almost every time we went out to eat she and I shared food so it’s not just a family thing. Do YOU share food - not like small plates - and if you do or don’t, why? (I’ll add a post script that I also hate to waste food.)
I don’t want to share my bagel and lox. 
Well, there ARE limits!!!