The assumption is that you work for the startup and have a critical enough position to earn equity, and therefore have more influence over its performance than that of a public company.
And "startup" doesn't have to be 3-4 people. I remember trying to recruit an engineer when Google was maybe 150 people. I tried to convince the engineer that Google was doing well and it wasn't as risky as it looked, but she decided to go work for Siebel instead.
In other words, don't waste money/time/effort on things where you can't make a big difference. Find an area where you can have a lot of leverage or expertise and put your chips there.
Hindsight being 20/20, for every "She turned down being employee number 151 at Google to work for Siebel LOL" story, there are 10,000 (maybe 100,000) "I took a chance with Pets.com and they still owe me 6 months of back pay" stories.