It is unclear from my reading of the gemara when Birkat HaMazon is required. As a question of Halacha that may be of interest to others and is certainly of interest myself....
1) What is the Halacha if one eats less than a kezayit of bread
2) If he fills himself on the rest of the meal is he required to say Birkat HaMazon
3) Does he need to then make brachas on the other foods he eats since they are not secondary to his bread?
4) Is he Yotzi Kiddush on Shabbos and YT if he eats less than a kezayit? (this issue comes up at Shabbos when people don't want to "fill up" on bread and at Wedding Seudot, etc. for the same reason)
1) If one eats less than a k'Zayis of bread, one does not recite Birkas ha'Mazon (Shulchan Aruch OC 184:6).
2) No.
3) Logic would dictate that one would have to recite a blessing on all the other foods, because they are no longer subordinate to the bread. However, the Mishnah Berurah (177:3) cites a number of Acharonim who wrote that perhaps since the way of the world is for people to establish their meals on bread and to recite a ha'Motzi on the bread to cover all the foods that will come within the meal, that it will also cover those foods even when a small amount of bread is eaten. The Mishnah Berurah concludes le'Halachah that if one wants to eat a number of foods but does not want to eat more than a k'Zayis of bread, he should not eat bread at all. (Alternatively, Rav Landy suggested that one make sure to eat a k'Zayis of bread at the beginning of the meal in order to cover the foods that come within the meal.)
(However, it could be that the question of the Mishnah Berurah is if his only intention in eating the bread is to exempt the other foods from their blessings. Perhaps if the person wants to eat the bread for its own sake, then even if he eats less than a k'Zayis, it would cover the other food. -M. Kornfeld)
4) One must have Kiddush in the place of a Se'udah. If he does not eat a k'Zayis, he has not eaten a Se'udah, and therefore he has not been Yotze Kiddush. (Mishnah Berurah 273:21)