The reason desktops are larger is air flow and versatility. A large case can hold more components and temperature control devices like fans and heat sinks. You have bays in a case so you can add many internal hard drives, multiple video cards, a vast amount of RAM.
Desktop Computers & Servers
Lenovo V530 Intel 9th Gen Core i3 Tower Desktop (4GB RAM/ 1TB HDD) 19.5″LED Monitor