My largest Heatwave tomato plant started from a seedling about the same size as the mystery one pictured above. I had bought a normal sized seedling to use in an attempt at upside-down tomato plant growing. As I transplanted it into the hanging basket, I noticed the very small plant growing beside it. I carefully removed it, planted it in a small container, and placed it under the lights for a month or so before transplanting it to an outdoor container. Meanwhile, the upside down plant turned out to be a horrible failure (but that's another story). The bonus seedling has since grown into a large plant with several tomatoes ripening on it at present.
Heatwave tomatoes ripening with basil companion plant