A little more detail.
So you have a couple of things to look at when you are talking about 12V wiring in computers.
Wire gauge, expected load, and connection standards.
6-pin PCIe is really a 5 pin connector. The sixth is sometimes run as an additional ground. Expected TDP of 75W
8-pin PCIe is really a 7 pin connector. The eighth pin is sometimes run as an additional ground. Expected TDP of 150W.
Now you might note that the 8-pin connector only has one additional power carrying wire, yet is rated at twice the amount of a six pin connector. Generally 6-pin connectors with 18 gauge wire have the capacity to carry upwards of 120W @ 12V before seeing an appreciable voltage drop. Similarly an 8-pin connector can be rated at 180W.
18...