In Siman ?? it says that it is not Derech Eretz to bite into something bigger than a k’beitzah.
A. Not derech eretz <> “numerous issurim” (and yes, I realize that you are not the one who said it involved “numerous issurim.”)
B. That may not apply to foods that are normally eaten in that manner (such as a bagel, for example).
C. It may also be something that is dependent on the style and custom of the culture. For example, I always tell my kids to cut their meat small enough to eat it in one bite (which fits in quite nicely with the S”A on the matter). However, I would never tell them to cut a candy bar with a knife in that manner, since the normal way of eating one in our society is to bite it off the end.
The Wolf