Sometimes a person will eat a Kabeiyah of bread at a Sheva abrachos or Shabbos table at the beginning, and then there’s Divrei torah and speakers or talking, etc. for 2-3 hours later WITHOUT EATING. Should one eat a kabeiyah at the end of 2-3 hours later in order to bench?
If you still feel satisfied from eating the meal, there is no need to eat more. However, because this is often hard to gauge, it is good advice to eat more before benching, as you suggest, relying on the majority opinion mentioned in the Mishnah Berurah according to which there is no need for a new Hamotzi under these circumstances (see Shulchan Aruch, Orach Chaim 184:5; Mishnah Berurah 17).