I've found some details. Tomatoes can be "indeterminate" which means genetically they can grow "forever". Some varieties are "determinate", which means apparently overall size of a life form can be limited genetically. Tomatoes can be determinate or indeterminate.
Determination in plants is controlled by the
apical meristem. The meristem consists of undifferentiated cells capable of cell division. As you'd assume, it gives rise to various tissues and organs of a plant and it's responsible for growth by producing essential plant hormones like auxins that flow throughout the plant. As long as this meristem is present, the plant will continue to grow. Cells that have differentiated into tissue no longer have these pluripotent, indeterminate properties.
In the case of the large tomato presumably the high nutrition and other cultural practices he employed simply let it rip beyond what poorer horticulture would result.
Agreed.
However I am still interested in the biology of poorly grown plants and animals vs well grown. Measuring genetic potential, if we can.
I believe it's easier explained with plants than with animals, since their biology is relatively more simple, less tissue types, organs, etc. I'm struggling to respond to this because I don't understand what genetic potential means in your perspective. For example, you and eye both have alleles that code for skin color pigmentation, relate your idea of genetic potential to that trait (or any of your choosing) so that I can provide you a well-informed response.