Here is the irony of using a big needle, in my experience. You think that a smaller needle would be less painful but in actuality it can be more painful. This is because the medication moves much faster and harder through the smaller needle which can cause a burning sensation within the muscle. (You can test this using a 10cc syringe and then using a 21g and an 18g needle to see which one squirts faster.) It is more difficult to inject a viscous medicine through a small needle as well, and it can get tiring. In expert hands, an 18g feels the same as a 21g when it pierces the skin and muscle.