Many times, artist renderings of spacecraft don't do it justice in terms of just how large these things can really be. While some spacecraft are larger than others, with some being the size of an automobile, many are monstrously larger than school buses.
For example, New Horizons was about the size of a Smart Car, but Juno it about as big as a basketball court. The International Space Station, on the other hand, is as big as a football field.
Sizes vary because different spacecraft serve different purposes. Obviously, those that carry people, like the International Space Station, need to be larger to accommodate for room. On the other hand, non-manned spacecraft can be smaller because they just need to house sensors and fuel.
In addition, many modern spacecraft are using solar arrays to power their equipment, which makes them wider than many older spacecraft that used nuclear power instead.
While pictures can be deceiving, NASA has tons of resources on their website that share accurate information about true spacecraft and space vehicle dimensions for your viewing pleasure.