Generally speaking baked goods where the primary solid ingredient is flour from one of the 5 species of grain should be hamotzi no matter what amount is eaten. One exception to this rule mentioned in the Gemara and Shulchan Aruch is called pas haba’ah b’kisnin, which is mezonos unless one eats a quantity that most people would consider a full meal when eaten with appropriate side dishes. The Beis Yosef explains the underlying logic is that pas haba’ah b’kisnin is a form of bread that has lost the primary quality that gives bread its special beracha; it is not usually eaten as the mainstay of a meal.
The Rishonim give three different explanations for what qualifies as pas haba’ah b’kisnin: a filled pastry, dry crackers, and the one relevant to our discussion is dough kneaded with sweeteners such as juice or sugar. According to the Mechaber if the sweetness is significantly noticeable it loses its status as regular bread, according to the Rema it needs a predominantly sweet flavor. Cake is the most obvious example of this form of baked goods. Some Poskim, notably the Maharsham in Daas Torah, write that if the majority liquid in the dough is other liquids such as juice and not water, the item needs not be extremely sweet to be mezonos. Based on this, many have come under the impression that there is such an item as a “mezonos roll”.
However there are numerous problems with this conclusion. Most “mezonos rolls” are made with reconstituted juice where the water is actually of greater volume than the concentrate. According to most Poskim only the concentrate counts as juice, and in fact the roll contains a majority of water. Additionally, these rolls are designed to be eaten as a sandwich for a meal, and fail to meet the underlying logic of pas haba’ah b’kisnin as a non-staple pastry; rather they are considered in Halacha to be regular bread. The Marsham himself clearly is only referring to a food that is not eaten as a meal because of its sweetness, because he writes that eggs and beer, while technically Mei Peiros, do not count as “other liquids” to render a pastry baked with them as the primary liquid a non-bread item because they are not sweet. Clearly it was never his intention to consider the presence or absence of water the sole qualifying factor to determine what beracha is said on baked goods.
So it seems to me that a mezonos roll or bagel must be hamotzi even if only a small quantity is eaten.
Many pizza shops feel they are better serving their customers if they can save them pizza that does not require washing and bentching, so they make the dough using apple juice or milk, in effect a “mezonos roll”. While Reb Moshe zatzal seems to have accepted this, but as we mentioned previously it is conditional on considering pizza a snack and not a meal. If pizza is generally eaten as a meal by most people most of the time, it CAN NOT be pas haba’ah b’kisnin and is always hamotzi, even for HALF a slice or less.