The one who decides if it’s cheating is your self feeling of accomplishment, because one of the things that makes kendamas interesting is the feeling when you challenge yourself within your own rule and you hit it. So if you do “cheat” but you feel, “Well I’m happy I did the trick regardless” then power to you… though I would guess you will still have that guilt feeling inside.
If bigger kendamas “easier”, then you can do harder tricks and in the end it’s still difficult enough right?