European wire gauge is not the outer diameter. It is the cross section of the conductor inside the wire in mm^2. It is the same system AWG uses (they are directly correlated) with the added benefit that the numbers make sense (10mm^2/AWG8 wire has 4x the cross section of 2.5mm^2/AWG14 wire, so a quarter of the resistance of the thicker wire and thus roughly double the current capacity).