The thing is, at that point going to a larger public company would probably net you similar returns and less of the headache (immediately liquid compensation that you can invest into other things, no tail risk of option value suddenly becoming 0, likely better wlb, etc).

Once it an "obvious" choice to join a startup then the valuation of the company is already close to fair, assuming you have about the same edge as VCs do when evaluating these companies. On IPO day it may jump up a bit from its last posted private valuation, but keep in mind once the company IPOs you're typically subjected to a 1-year lockup period, during which the value of the company could change drastically.