That table is AWG to external diameter, not necessarily the internal wire cross sectional area. It's best to go with the conductor cross section area for current calculations.
20 AWG is ~0.5mm2
18 AWG is ~0.8mm2
In terms of cables, defiantly don't judge them by their cover! I've seen supposedly beefy cables visually be tiny internally, the jacket is much cheaper than the metal inside!
Also, I'll also throw this in, don't use Copper Clad Aluminium (CCA) cable either. It's a horrible conductor at DC and is a pain when it ages.