Similar to @user6591 and @Gavriel's idea (above)
It seems to be a natural result of fulfilling the correct way of baking bread on Shabbos
Shulchan Aruch Harav 167:4
Among the guidelines of proper conduct is not to cut off a [very] small piece of bread, because then one appears miserly. Nor, should he cut off too large a slice, i.e., one larger than an ...
If he is "borrowing" 2 Challas which he cannot eat he cannot make a Brocha on them and it is as if he is only holding the Broken bread as he can only make a Brocha on his own bread Mishna brura Orach Chaim 161,5: 21:
ודע דכהיום שהמנהג שכל אחד מברך לעצמו ברכת המוציא לכאורה לא שייך כלל דין זה אלא כל אחד יברך על פתו שאוכל [מחה"ש]
One does not need to take ...