Let’s look at indoor cabling.
Here are 3 random non-audiophile power cables I have around the house. All 3 cables are certified for Australian use.
Specimen A, this is a typical power cable you can buy around electronic stores. If you look at the specifications, every core has a cross-sectional area of 0.75 mm2. This is the most common type I am aware of.
Specimen B. This cable is rated for high temperature environments (hence the notch). This cable has a core cross-sectional area of 1 mm2.
This cable is supplied by Cisco Meraki - and I love the bling bag.
Speciman C. This has a cross sectional area of 0.824 mm2. It also has a thicker insulation.
As you would expect. The higher the cross sectional area, the heavier the cables become. All cables are rated to be capable of delivering 10 Amps at 240 Volts. The difference with A vs B/C? The latter can do the same in harsher environments!
Even with these indoor cables, they are not all built to the same standards, fit and finish. You do pay for what you get for.
So why is it OK for these leads to be 0.75 mm2 but in general it’s not OK for in wall cabling to be 1 mm2. The answer is current. If you use 1 mm2 in wall cables and it goes to a 2 outlet GPO, or if that is split into more GPOs. The total capacity will be exceeded. In the very worse case scenerio this will cause an electrical fire.