There is a technique to donning a properly fit full face helmet and your ears will never know anything happened. I've seen some people put a full face helmet on top of their head and simply push down. Guaranteed, that helmet is WAY too big for that head if it goes on that way.
Pulling outward on the staps is good advise. It takes a bit of practice and hand placement on the straps can make a difference. You want to grab the straps close to the helmet.
Also, most find that bringing the helmet somewhat from the rear, rather than straight down on the head, works better. You start with the helmet tilted up wards, move it onto the back of the head, then rotate the helmet forward as you pull it forward and down.
Sounds complicated in text, but once you get the hang you'll do it in your sleep.
If you full face helmet will come straight down on your head without bending your ears, it's probably too big.
If you ever get the chance to watch a professional rider don their helmet, pay attention to how they do it. You have to watch closely though because they do it quickly. Just like they ride.