Every picture has two parts: the objects (form) and the space around them. Handling that space well is what makes a layout feel clear or cluttered.
Negative space counts. The empty areas are shapes too, and they need as much care as the objects. Island World is mostly sky, and that empty sky is what makes the islands feel like they float.
Distance shows relationships. Things placed close together look related. Things far apart look separate. Tight spacing feels busy; generous spacing feels calm.
Overlap shows depth. When one shape covers another, it looks closer. Making distant things smaller and paler adds to the effect.
Layers organize a scene. Our worlds are built in three layers: far hills, near hills and the ground in front. When each layer is clear, the eye knows where to go.
Leave room. White space is not wasted. It gives the important parts room to be seen, on a canvas or on a web page.
Try it: find a crowded page or screen near you and imagine removing one element. Often it gets clearer, not emptier.