The reason that XT40i was designed as 4.0mm
2
was so that it was able to match a certain price point, and a bigger
cable would have meant a more expensive one. American Wire Gauges
represent a range of cross-sectional areas and so really, they are not
the best way to specify speaker cables because, for example, 12AWG
ranges from 3.3-4.1mm
2 so XT40i is at the top end of this and is
nearly 11AWG.
If
you would like a true 10AWG speaker cable featuring QED AirCore
technology (which is even better then X-Tube), we so already have one,
called
Genesis.
Also there is
XT25 Bi-Wire (which can be used in single wire mode) giving 5mm
2, which is a high 11AWG (nearly 10AWG which would be 5.2mm
2).