The answer is exceptionally simple. If you are equally good at 6max as at ring (this is NOT a given, but for the sake of argument I'm assuming it) you will see roughly 40% more hands and therefore make 40% more profit. It's as simple as that.

If you are a 13/7 nit and can't loosen up for the life of you, stick to full ring. But if you can use position well and are happy letting your VP creep above 20% and your PFR 10%, then you should be fine at 6max.

There does need to be a proviso - 6max is swingier, takes more balls and can fuck you in the arse if you lose concentration or go on tilt. So you need to be even more well-disciplined and have a solid fundamental game before you can rely on making the sort of $$$ there thhat should be possible.