In the textbook Asset Pricing by John Cochrane, on p. 25, it says:
"This prediction holds even if the payoff $x$ is highly volatile and investors are highly risk averse. The reason is simple: if you buy a little bit more of such an asset, it has no first-order effect on the variance of your consumption stream."
What does "first-order effect" mean here? Why would buying more of such an asset have no first-order effect on the variance of consumption stream?