Is my football helmet to big? or is it bc of the missing pads?

so im a freshman in highschool and we got our helmets for football a few days ago, and the ear pads in the helmet are to small and the coach sed he'd get me a diff pair, and i havent completely got my chinstrap in place yet, but the helmet slides back on my head causeing the facemask to go up my head so looking through the bars and not the given hole to look through. im wondering what i can do to stop my helmet from doing that, is it bc of the small pads the un settled chin strap or is it just bc the helmets to big? i wear a medium, im 5 4, 105 lbs, not very strong. what can help it from sliding up and making me look through the bars?