Helmet sizes differ depending on their purpose, the specific model, and also — the manufacturer. Letter markings (for example, size M, L or XL) on other types of helmets, such as a sports or construction helmet, don't mean that these same sizes will be identical for a military helmet.
That's exactly why, when choosing a military helmet for yourself, you need to start not from the letter marking on it or in the description, but instead to be guided by the numerical range of head circumference in centimeters. This way you'll be sure that a specific helmet model of a specific manufacturer and a specific size will suit you.
Using the example of our helmet MICH™ — for each size we indicate the head circumference in centimeters:
Size L corresponds to a head circumference of 56–59 cm
Size XL corresponds to a head circumference of 60–63 cm

See our helmets and choose the size you need: https://ukrainianarmor.com/shop/sholomy/