There are a few ways to achieve balance in photos.
The first is through symmetry - where you have equal size subjects on either side of the photo. This creates a static, solid look with little movement.
The second is to place dissimilar size objects on either side, but to use the center of the photo as a balance point in an asymmetrical composition. Just like an adult and a child on a teeter-toter, the adult has to be much closer to the center balance point for the child not to be held way up in the air the whole time. If you have a grouping of objects on one side, you'll need something further out on the other side to create balance. An asymmetrical composition creates a sense of movement and action, even if the subject is a stone.
Exercise - collect a bunch of rocks, some similar size and some dissimilar. On the sidewalk or other handy surface, try creating several symmetric and asymmetric compositions, taking pictures of each.
This exercise is based on exercises in Drawing on the Artist Within. This is an excellent book for learning about creating art - the basics are taught through drawing but are applicable to all art forms and just great for learning to be creative. I highly recommend it. (click on the cover image to purchase from Amazon.com)