I think a lot of it is variety specific, and some of it is fruit type-specific.

On this year's Juliet, the fruit, on average, decreased in size as the season progressed; this was also true of the Rutgers. I suppose I'd interpret that as the root system, available nutrients, and water not quite keeping up with the plant's fruit load. Maybe it's true of all tomatoes....

This year's Juliet was also notable in that every single flower produced a tomato (except for a couple of blossoms that were lost to accidents and a young truss that a hornworm ate). The volunteer grape I grew for three years was almost as consistent. That was not the case, however, with the German Queen, where many (most?) of the fruit never matured beyond the pencil-eraser stage.

Some beefsteaks seem to have a single "king fruit" in each cluster (perhaps the first flower to bloom?) which grows much larger than its siblings.

Things to think about keeping track of next year....

On this year's Juliet, the fruit, on average, decreased in size as the season progressed; this was also true of the Rutgers.

This was true for me last year, too for Rutgers, Marglobe, Ramapo and Moreton. It gets pretty hot here as the season progresses so that may have been the problem.

I would guess it has to do with the demands of a maturing plant on the soil, like missing says. Also, I would guess that even though there are inderminates out there, that there is still a prime window in their lives. It sure is hard to pull an aging,healthy plant, but I have seen it firsthand. It is hard to beat the first couple tomato sets when it comes to size on medium to large slicers.

Take care,
Travis

Hundreds of variables can affect it - age of plant, over-all health of plant, container or in-ground, stem size, primary or secondary or tertiary etc. stem, number of fruit on the plant, number in the cluster, genetics, weather at fruit set, weather during growth, available soil nutrients, available soil moisture, sun exposure, consistency of soil moisture, pests or disease issues and the timing of them, treatments used for pest or disease problems, etc., etc., etc. to name just a few.

Dave

We can agree on the obvious issues affescting fruit size but it all boils down to the plant's ability to metabolize and move vital nutrients to the developing fruit. If one could do a regular plant tissue test (similar to a human blood test) and correct for deficiencies there is good reason to expect better consistancy in fruit size throughout the growing season. Nitrogen is probably the nutrient in greatest deficit as the season progresses. However, inadequate amounts of even trace elements like boron may affect overall plant health.

One thing that wasn't mentioned is pollenation. the number of seeds in a tomato are directly related to how well the plant is pollenated. the more seeds, the bigger the tomato. If you're seeing all of the tomatoes are small, it might be a nutrient deficiency. if you're seeing that the early bloomers are small, or late bloomers are small, it might be water, or light, or again a nutrient deficiency. but if you're seeing some small and some large at the same time, it might be a pollenation problem. if your tomatoes are outside, maybe the bees can't reach some of the flowers. if your plants are in a greenhouse or indoors you should try using the electric toothbrush technique for pollenating tomatoes. google it for specifics on the technique.

the more seeds, the bigger the tomato

You might want to qualify that statement just a bit. The overall number of seeds does not always correlate to the size of the fruit. Varietal genetics and environmental factors play a major role.

Overly simplified, true, but if the statement was accurate as stated then cherry varieties would be virtually seedless while beefsteak varieties would be little more than balls of seed.

Dave

As an engineer doing my best to observe, but also terribly
lacking in a comprehensive understanding of genetics I can report as follows:

1)My wife and I have 3 offspring. The differences are remarkable!

2)Having grown rows of the same variety sourced from seed from the same tomato I note remarkable differences in
plant performance, fruit size, and quality. Hmmmmm.

EdJekins, and others who may not know '
please be aware that Tomatoes are self pollinating and bees are NOT required at all .

Are all the eggplant on the same plant the same size? All the green beans the same? All melons? All roses or cukes or ears of corn, or daisies, or potatoes, or squash, etc? No. So why would one expect tomatoes to be any different?

Dave

I have four children ,my youngest daughter is bigger than her older sister.They are not plants though.

Because thy're mass is not constant

As an engineer doing my best to observe, but also terribly lacking in a comprehensive understanding of genetics I can report as follows:

1)My wife and I have 3 offspring. The differences are remarkable!

Unless your 3 offspring are identical triplets, that is not an apt analogy.

Some beefsteaks seem to have a single "king fruit" in each cluster (perhaps the first flower to bloom?) which grows much larger than its siblings.

This is exactly what I am seeing and it seems to be the first flower to set fruit.

I have four children, my youngest daughter is bigger than her older sister.

Right, but with tomatoes on the same plant, they are genetically identical unlike your children (unless your children are identical quadruplets that is).

Yumtomatoes,

My eyes and my mouth suggest to me that all the tomatoes on one plant may not be GENETICALLY IDENTICAL!!!

Furthermore, I have saved and kept segregated seed from two
tomatoes sourced from the same plant. These were planted in rows side by side the next spring. There were observable differences in row to row performance, as well as
plant to plant performance in the same row.

Of course, there is the rare chance one or both could have been externally cross pollinated--not very likely.

On only one occasion have I gone to the trouble of bagging
a blossom cluster before they opened, then saved the seed for future use. Though I did not do side by side row comparisons from seed collected from more than one tomato
I did see plant to plant variation from single tomato sourced seed.

I have a suspicion that there are trivial genetic variations in all seed from the same fruit and major variations between some of the seeds. I can't otherwise account for what I have observed.