I don't think this is a valid argument. When startup employees lose their jobs it's usually because the entire company has gone under. Motorola may be doing massive layoffs, but they're still around and they still have plenty of employees. If you're good enough to be right for a startup, and your current BigCo isn't so bureaucratic that they do their layoffs strictly by seniority, then if half the company gets laid off, you stand a pretty good chance of being in the other half.

What's really important is just to have enough faith in your own abilities that you don't consider losing your job to be a big deal. Of course, you should still either go in with enough savings to cover living expenses for a month or two, or join a company that has enough money in the bank that you can acquire those savings before their money runs out.