Pizza delivery drivers usually get a set amount for each delivery, like 75 cents or something (maybe more nowadays). If you order during a really busy time they are probably not cutting your pizza (but they probably are at least making sure you have the correct number of pizzas and bringing you the cheese and pepper packets); but if you order late at night they most likely are cutting your pizza because fewer employees working at that time means everyone does a little extra.
Also, 8 pizzas are a lot harder to carry than 1 because only 4 pizzas fit into one of those heated poach things and carrying 2 of them is a bitch.