Invisalign tray is bigger than my teeth. is this normal?

I've been wearing Invisalign for one year now...starting these past two months my trays have become very visible on my teeth because they are actually bigger than my teeth...on some of the teeth only..three or four it is very visible that the size of the teeth is smaller than the actual tray...I have asked my orthodontist about it and he said that this is normal as according to him the difference comes from extra material added to push the teeth upwards..