It's smart to question that assumption. Ski hip protectors fit much more snugly than regular clothing because they need to stay in place during a fall, so your usual XL might not translate directly. The XL in hip protectors typically fits a waist of about 100–112 cm and hips of 112–120 cm, but sizing varies between brands, so always check the specific chart.
The key is pad placement. If the protectors are too loose, the pads can shift off your hip joints during a fall, leaving you unprotected exactly when you need it most. If they're too tight, they'll be uncomfortable and may ride up. For XL frames, you want full coverage across broader hips without the shorts feeling restrictive.
Measure your waist and hips at their widest points before ordering, and if you're between sizes, it's usually better to size up slightly—you want a snug fit, not a suffocating one.