What camera do you use? Maybe we can help improve your pics. The main problem you'll have with small subjects (relative to the frame) is that your camera will not be able to meter properly, especially if it is a p&s. having a small bright subject in contrast with a dark background will most likely result to an over exposure of the subject. on the other hand, a small dark subject on a bright background will most likely be underexposed. Detail is important in macro, and when you crop too much, the loss of detail due to the lens or sensor will become more obvious. And composition in not just where you put your subject on the frame but is also how you "look" at the subject. Cropping will let you move the position but it can not let you change the point of view.

This was one of my pictures when I was starting up with macro. I used a non macro 50mm lens (didn't  have a macro lens yet) then cropped. I didn't use flash to keep the contrast right.