Why is it improper to talk until after one has eaten a kezayis (of bread)? Does this apply by other foods, as well?
Even the smallest crumb of bread requires a bracha rishona; therefore, the bracha of hamotzi takes effect as soon as the smallest crumb of bread has been ingested. Moreover, there are times that a person does not eat a kezayis at the beginning of the meal, but only later on.
Since the blessing of Hamotzi exempts all other foods eaten from a bracha therefore one should eat lechatchila an amount of bread that is considered significant i.e. a kezayis. This reasoning does not apply to other foods.
O:CH 167:1 Magen Avrohom 7, ibid Dogel Mirvov, ibid Machtzis Hashekel 16, ibid Mishnah Berurah 35