If you changed the amount of light, AND changed the amount of water each plant received, you wouldn't know if the bigger tomatoes were because of the light, or the water! So it's absolutely important to have everything be as close to identical or "controlled" as you can get it.

Once you've done your science fair experiment, consider repeating it if you have the time! This is very impressive for science fair judges, and really helps to make your results convincing. If you conduct your experiment and the results just don't seem right, you might consider reviewing your experiment idea, and possibly even modifying your experimental design so that your science fair experiment will produce accurate results.