:D That helmet has seen a lot of uses throughout pen and paper RPGs (not sure if it ever actually served the purpose of protecting Leafhelm's head from anything). It is quite funny to see in how many ways an oversized helmet can be diverted for useful purposes ;)
Quite obviously it is way too large for Leafhelm and was probably never made for a pony to begin with (Leafhelm dug it up near some ruins when he was a foal and that's when he got his cutie mark).