Where you run into trouble is once you start loading it up with like 2 or 3 different meats plus bacon and slathering cheese on like there's no tomorrow, and then including loads of mayo and mayo-based sauces and toasting it with butter and so on and so forth. If you go into a sandwich shop and this describes half their sandwiches, it's because that is what their customers are looking for, but you can absolutely get a proper sandwich by asking.

A decent sandwich consisting of meat, some fat (either avocado or cheese, you don't need both unless you're making an avocado and cheese sandwich without any meat), and some good quality veggies on sliced bread, not rolls, is still going to come out pretty big, but it will fill you up and get you through the afternoon. More importantly, a good 50% of the overall thickness will be from the vegetables if you make it right. If you buy loafs, buy oval-shaped loafs as well, the outer parts can be used as toast and the inside will make solid sandwich bread.