Relative leaf size, in fact, is so important that it warrants a term all to itself: "texture." That is, the size of a plant's leaves (relative to what's around it) influence how its texture will be perceived: relatively large leaves give a plant a "coarse" texture (the zinnia, here), while relatively small leaves create the impression of a "fine" texture (the cosmos, here).
In addition, we tend to perceive the texture of plants with a lacey leaf form, such as cosmos, as being fine.
