That would probably be true if all other conditions held constant.
However, Silicon Valley can pay handsome money because many companies located there run well and outperform their competitors for a fertile global market. And those companies can beat their competitors because (at least part of the reasons) they have the best talents available. If the skilled engineers from China, European, India all went back to their home countries, would the competitiveness of those SV companies be no different relative to their foreign competitors? If companies in SV were losing its advantage to another tech hub outside the united states, would they still be able to pay so well? I doubt that.
More people would mean smaller pieces if the size of the pie held constant. But what if there are also fewer people making the pie?
In reality, it's even more complicated in that the contribution of the pie making is not proportional to the headcounts either.