They aren't big - and they are not small. Others have small leaves. Some daylilies - the small ones, have leaves that are almost as thin as grass. It is hard to se here, but the daylily in back has fine textured leaves, and the Calla lily has somewhat medium leaves, although they are a LOT larger than the daylily foliage. This looks much more interesting than if everything was the same size.
Plants also have different shapes .
Some are very straight and upright, like the cannas. Some make big rounded shapes, like hostas. Others, like daylilies, are shaped like fans. You want at least three different shapes in your garden, as well as three leaf sizes. You may not always be able to tell a plant's shape, or even a leaf's size from a catalog picture, so ask your mother or a friend who gardens for a bit of help here.