Both my tomato and watermelon plants look great and the tomato plant is producing quite a few tomatoes but they never get any bigger than halfway between a golf ball and a baseball. Also, my melons have grown to about the same size and have stayed that way for a couple of weeks.
PS: The watermelons are Sugar Babies. The tomatoes are 2 types in different parts of yard-Big Boy and Best Boy
Have you been fertilizing with an acidic fertilizer? If so, the plant is extremely happy, and the 'fruits' aren't. Consider switching to a well-balanced fertilizer such as 10-10-10 or to a "bloom booster" 15-30-15.