Wear your suspender belt at your waist. It should be tight, not uncomfortable, against your body. A suspender belt looks much smaller than it should be. But don't let that discourage you.
The measurements given with the size refer to your body measurements.
The item, suspender belt itself is smaller because it needs to fit snugly around your waist and body to stay in place.
For example, person's waist measures 90 cm, and this person usually wears a size L suspender belt which waist measurement is approx. 60-70 cm depending on belt style and type of waist elastic. However, size M also fits, though it's not ideal, but it's easy to fasten. This person can also wear size XL which is quite comfortable, but the stockings might pull the suspender belt down because it is not tight against the body.
If you prefer to wear the suspender belt lower on your hips, or don't like a tight fit, you might need a size larger than what is written in the size chart.
For all bodies that don't have a prominent waist, my recommendation is to buy a garter belt up to two sizes SMALLER. Confirmed by a customer.