If you do the math, increasing your contribution by a couple of percent hardly makes a dent in your pay. For example, if you make $50,000 per year, a one-percent increase in savings works out to be approximately $42 extra, pre-tax savings per month. If you make $100,000 per year, it's approximately $84 extra, pre-tax savings each month. You could likely make small cuts to your monthly budget (pack lunch for instance) that would easily allow you to accommodate a small increase.
On the other hand, even a small amount of money can grow much larger with time and the benefits of compounding.