In the picture, the plant with the smaller, brown fruits is far thinner, and taller, and has small yellowish leaves. It gives me the impression of nutrition lack. So Bamboo is probably right. Do you have access to manure? Side-dressing with that will improve the health of the plant. If not, you can apply a balanced fertilizer as a side-dressing, following the directions on the product label. Or if you have a soluble fertilizer, like Miracle Gro, mix according to the label, and water it in once every two weeks.
Once the soil/plant is healthier (depends whether you use manure/bagged fertilizer) the fruit should become normal. If it doesn't, hat means that the other plants actually are different varieties. There are many kinds of eggplant, and people make mistakes.
I would also take the weeds out back past the area, at least as far as the plants are tall, preferably more, and add mulch (not woody mulch, like bark or chips). Something like straw, or grass clippings will do. Apply in a layer light can't get through, to help block the weeds and preserve soil moisture. Keep the plants watered, very necessary for a fruiting plant, and be sure to pick fruits as they ripen.