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).
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).