Small tanks are great for when you're not too invested in having a very diverse ecosystem in your aquarium. They're extremely maneuverable, and you'll be able to get a lot of fish for a cheap price that can thrive in smaller tanks. However, it's hard to fit filters in small tanks, and you have to clean them more frequently.
Larger tanks are for when you want an extremely eye-catching display, with large and colorful fish. It's also recommended for people who invest lots of time in choosing fish, and wish for a diverse aquarium full of different species and breeds. These are more bulky and take up space, but you can fit filters into them. They also stay clean for longer periods of time.