Post History

This is directly addressed in the Shulchan Aruch, Orach Chayim 168:6:

> פת הבאה בכיסנין מברך עליו בורא מיני מזונות ולאחריו ברכה מעין שלש ואם אכל ממנו שיעור שאחרים רגילים לקבוע עליו אע"פ שהוא לא שבע ממנו מברך עליו המוציא וברכת המזון ואם מתחלה היה בדעתו לאכול ממנו מעט וברך בורא מיני מזונות ואחר כך אכל שיעור שאחרי' קובעי' עליו יברך עליו ברכת המזון אע"פ שלא ברך המוציא תחלה ואם אכל שיעור שאחרים אין קובעים עליו אע"פ שהוא קובע עליו אינו מברך אלא בורא מיני מזונות וברכה אחת מעין שלשה דבטלה דעתו אצל כל אדם:  

> Pat Haba'ah Bekisnin,[^1] one blesses on it "who creates kinds of nourishment (borei minei mezonot)" and afterwards the three part bracha[^2]. If one ate a quantity that others would usually use to establish [a meal], even if he wasn't satisfied by it, one blesses "who brings forth (hamotzi)" and Grace after Meals. If in the beginning his intention was to eat just a little and blessed "who creates kinds of nourishment" and ended up eating a quantity that others establish [a meal] upon, he should bless Grace after Meals even though he didn't bless "who brings forth" in the beginning. And if he ate a quantity that others don't establish [a meal] upon, even though he is establishing [a meal], he should bless only "who creates kinds of nourishment" and the three part bracha, as his own mindset is irrelevant compared to [the mindset] of all people.  
<sub>(text and translation from Sefaria)</sub>

This is known as the concept of קובע סעודה, "koveah seudah", establishing a meal.  
The example I've always heard is with pizza. If you're grabbing a slice out of the fridge, that's not generally considered by the average person to be a "meal". If you're sitting down and eating half a pie, though, pretty much everyone would agree that that's enough to be worth saying Birkat HaMazon (Grace after Meals) afterwards, since that's definitely a meal (and ideally you would have washed and said HaMotzi beforehand).

There are a few foods where you say Mezonot beforehand but don't bentch (say Birkat HaMazon) or say Al HaMichya afterwards, such as rice. The "Pat Haba'ah Bekisnin" concept only applies to food made with one of the five grains (wheat, spelt, rye, oat, and barley).

Note that "koveah seudah" only applies to "Pat Haba'ah Bekisnin", which is made from the five grains but doesn't look like or function as bread. If you've got a challah, pita, baguette, sourdough, or the best thing since sliced bread, they aren't considered Pat Haba'ah Bekisnin. When eating actual bread, that looks like and functions as bread (and is made from one of the five grains), you say HaMotzi and Birkat Hamazon after a K'zayit or more. (Shulchan Aruch, Orach Chayim 184:6) 



[^1]: Non-bread bread-like foods, such as cookies, cake, and pastries.
[^2]: Also known as "Al HaMichya".