If we take pg's 95% literally and define YC's current contribution to a startup as 100%, then if they give 3x as much money their contribution to the startup becomes 110% of what they contribute now. So they would demand 10% more equity.
You're right that we wouldn't take too much more equity. (In fact, the last time we increased the amount we invested, we didn't ask for any more equity.) But if we invested more per startup we'd tend to become more conservative. We'd think twice about investing in some of the riskier projects, and that's the last thing we'd want.
