LEGO is pretty generous. In all of their sets, they have some extra pieces that are left over after following building instructions, but LEGO only gives you very small extra pieces, like studs, connectors, half bushes, etc. LEGO almost never gives you large extra pieces. If you have a large piece left over after building a set, check through the instructions again to see if you missed putting that piece in the model as part of the steps. In recent LEGO sets, each step has an inventory listed for the step. To avoid leaving pieces out while we build, we like to find the pieces for each step and make sure we use them all before moving on. When we have a large piece like a full brick or beam left over, we scan the inventory listed for each step to look for the type/color of piece we have left over. Usually, we find the step we overlooked that piece. It has happened to us many times in different LEGO sets we build.