Let me clarify
A sandwich refers to when something which goes with bread is placed between any two pieces of bread (hence, ice cream or Cheerios on bread is not a sandwich). With this definition, a hot dog would seem to be a sandwich.
However, there are certain "sandwiches" which are קובע שם לעצמם, have there own name, and are therefore not sandwiches. Two great examples of this are hot dogs and burgers (if you would slice up a hot dog and put it on white bread, it would be a sandwich). A hoagie or sub, while it has its own name, retains the sandwich name as well (with the exception of something like a meatball sub, which is just a sub and not a sandwich)