Strap length is the spec to check
Many belt bags top out around 45 inches of strap, which is fine at the waist and tight across a jacket. If you plan to wear it crossbody over outerwear, verify the maximum length rather than the bag dimensions.
Leather versus technical fabric
Leather looks better with normal clothes and weighs more. Nylon wins for sport and rain. For a bag worn against the body all day, leather also stops feeling clammy in a way coated fabric does not.
Capacity, honestly
A belt bag holds a phone, a wallet, keys and little else. Anything more and it bulges, which defeats the shape. Treat it as pocket replacement, not as a small backpack.
