Wire gauge hasn't changed in our lifetimes.
A given wire gauge will have the same total cross sectional area of copper, and the same weight of copper per foot, even if the diameter varies. I.E. a solid copper wire will be smaller in diameter than a stranded wire of the same gauge. If the weight of copper per foot is less for the same gauge, you're being ripped off.
Yes, insulation thickness varies between manufacturer, insulation type and batch.
I just installed some 2/0 19 strand copper cable as a ground here at work, and it doesn't look much thicker than my #2 copper welding cable, because that has well over 100 strands (so there is more airspace in the middle) and thick rubber insulation (versus the thin plastic on the THHN).