Read above, it can change in size.
Studies (I know people here don't like that word, so let's say a story I heard on the internet) show that muscle activation is increased by 20% if the person thinks about contracting the muscle while performing an exercise. So being able to focus on what you're doing and contracting the muscle plays a huge factor. I'm a big fan of selecting exercises where you can't cheat or it's very hard to cheat (so you'll just do it properly). Exercises like pull ups deadlifts. You either complete each rep properly or it's not a rep.