The sea serpent in the next drawing looks like a huge monster because it rises from the water close to the horizon line. Instead of the sea, one could imagine also an endless sandy plain or a vast meadow.
Exercise: Make up a drawing with a horizon (water or land) and place one or more figures near or far away.
An impressive example of how gigantic something can appear when drawn close to the horizon is “The Giant” by the Spanish artist Goya from 1814-1818. Down on the land, you can vaguely see some buildings, or perhaps the remains of a battlefield. The horizon line is hidden behind the low hill in the distance. The giant sits far away with his back to us, contemplating a day gone by, perhaps resting from a battle. His body and face are dimly illuminated by a low sun in the early evening, where we also see the crescent moon in the upper right. We look down at the landscape, but we look up at the giant towering far above our horizon/eye level.
The last drawing is an example of how in real life all kinds of objects often hide our viewing horizon (the red line).