Two psychologists studied nearly 1000 tips for restaurants, hair salons and with cab drivers. The larger the bill, the smaller the percentage tip. This is consistent with a reciprocal “payment for service” model. You pay the waiter enough to get the job done, but you don’t feel he has to work much harder to bring you a more expensive entree. Or you might simply be feeling poorer, the larger the bill.
Note that the effect levels off for sums larger than $100. After that point larger bills don’t lead to smaller tips in percentage terms. Servers also get bigger tips when they split the bills for large groups. Read here for more detail. Other research shows that servers get bigger tips if they resemble or can mimic the customer.