What type are you growing?

Gotta be the soil. Being weedlike tomatoes grow almost anywhere. But they grow best in well drained soil loaded with organic material. I've found that well aged compost used as a mulch really improves the soil and tomato production. A little fertilizer doesn't hurt either as long as you don't overdo the nitrogen.

Thanks trudi and jbann,
They are in pretty good soil. They have mostly been the sun sweet cherries and the sweet 100's. Is this just their nature? Should I have pruned them back when they were younger?

Oops.....I meant "sun sugar". They are orange and incredibly sweet.