Why is the hourly rate for smaller jobs sometimes more expensive?

For a trades business, each customer (that's you!) costs a certain amount before any work is actually done.

This includes costs related to advertising or marketing, plus the time taken to arrange the work or the quote, plus the travel time and expense to your home.

This amount is usually static and doesn't vary much except perhaps in the distance of your home from 'home base'. So, in a bigger job this cost can be averaged over the whole job (a lower percentage). With a smaller job the additional costs with make up a larger percentage and therefore increase the cost more significantly.