The Shulchan Aruch writes (170:7; listen to audio for precise citation) that one should not eat a large piece of bread the size of a Ke'beitza or larger as doing so appears gluttonous. From the Kaf Ha'chayim's discussion of this Halacha it emerges that this applies even to taking small bites from a large piece of bread that one holds in his hand. One should cut the bread into smaller pieces before eating it.
Many conventional eating habits today appear to violate this Halacha. When one eats a sandwich, pita or piece of pizza, he takes a large piece of bread in his hand and eats directly from that large piece. The work "Mishneh Halachot" mentions that he does not allow into his yeshiva students who eat pizza, since this violates the Halacha mentioned in the Shulchan Aruch.
It is not clear why most people today do not make a point of cutting their bread into smaller pieces before eating it; it would appear that one should, indeed, be careful not to pick up an entire sandwich, pita or slice of pizza to eat from it, and should rather first cut it into smaller pieces.
