Sometimes less is more...
Photographers are often advised to move closer to their subjects in order to fill the image frame because too much negative space around a subject will lesson the photo’s impact. This rule works well when what you are trying to say is expressed primarily through your point of interest. But it doesn’t work if what you are trying to say is expressed by a relationship between your subject and the surrounding space.
For example, if you are shooting an animal, you may want to back up and show habitat, or the environment of a particular flower, or it's relationship to other flowers. You may want to use a small subject with a large surrounding area to show scale and help to define that subject. When shooting a portrait, you might want to show the person’s environment too. So, the size of your subject in the frame should be determined by what you are trying to say, the story you want to tell, not by any rule! If in doubt, shoot your subject both ways, work it!
Here are some examples of photos I've made where I didn't choose to fill the frame. Try this the next time you are out shooting. Be sure that your subject is strong enough visually for this type of composition, and that the environment you are including adds to the scene.