Have you ever worked as a waiter or a delivery driver? Do you have any idea how averages and percentages work?
Tips on the large orders make up for tightfisted people who think its ok to derp around and tip a flat fee rather than a percent. Also, the larger the order, generally the more items that are brought out to you, hence the waiter is doing more work per order.
No where is that more important than being a waiter or waitress where it is completely 100% legal to pay below the minimum wage and their entire earning potential comes from tips. For delivery drivers that get paid more than minimum wage the tipping is debatable, but their pay still isn't that good and they have to deal with extra wear and tear on their vehicle.
Boo hoo, if being a delivery driver is such a bad deal, maybe they should have paid attention in school and learned something more useful. Face it, chauffeuring pizza is not a real job.