Often, the additional work that is required relates to the baggage. As Angus mentioned, there's the pre-board screening that the passengers go through, and then there's the baggage screening. For many of the domestic flights you're talking about, there's no baggage at all: it gets carried on.
For international flights, there are usually large amounts of baggage and more than one piece. All of that baggage has to be screened, as do the passengers, and often, with the passengers who are travelling internationally, there is a lot more baggage that they're carrying on their person.