When I bought my CZ82 it came with one thick and one thin. I guess that's how it was intended. I'm guessing the thick stayed in the gun normally when holstered and the thin was the extra mag in the holster with the gun since the thin would take up less space. Now, when you order surplus mags you get whatever the vendor sends you, thick or thin.
If you buy new CZ83 mags, the thin is 13 rounds .380 (12 rounds 9x18) and the thick (plastic bottom, not metal) is 10 rounds .380 (9 rounds 9x18).