No one has mentioned fertilizer. I guess because 'fertilizer' has somehow gotten a very bad reputation for being artificial no matter synthetically or organically produced.
Plants have to have very precise amounts of chemicals with which they use to make their own food, roots, photosynthetic growth that makes food for the plants, seed production and obviously fruit production. Soil does not come with these chemicals/fertilizers naturally. In self sustained ecosystems, there is little to none of these chemicals in the soil until something dies. That helps to maintain the system and limit population and subsequent needs of these chemicals.
The chemicals necessary for photosynthesis which makes the food for the plants to use as energy, also are responsible for the size and vigor of the plants and the size of the reproductive parts of plants; such as size of tomatoes.
The amounts of fertilizer necessary are found through soil tests and the ability to recognize the needs of your plants by how they look. And the sooner the better. When someone gets to the point of 'knowing' without soil tests what their plants need via experience and remembering the formulations already added, what kinds of organic material that has been added, what the symptoms look like by the plants showing deficit, excess of chemicals (nutrients) one is able to manage the chemistry of their soils to optimize vigor and larger fruits.
The manual process of taking reproductive growth off the plants; flowers and fruit/seeds is an important way to add more energy back into the plant and its vegetative growth. Cutting flowers off even before they open is excellent. You enable more energy into your plant to get larger, more vigorous, store more food/energy with which to produce even more flowers (especially annuals). To get more fruit/larger fruit you allow the flowers to grow into seed/fruit and select certain fruits you want to grow large. If you have a lot of flowers thus a lot of fruit you'll get smaller fruit if there isn't enough room to grow larger and if there isn't enough chemistry to put energy made by the plant into larger fruits. Taking off flowers and/or fruits will send that energy into the seed making/fruits and vegetative growth that are left.
If there are the proper chemicals, lighting and water to continue the photosynthesis you won't have to worry about pruning reproductive growth. Regular watering, regular temperatures, regular vigilance with chemicals will make fruits the size they were genetically designed to make.
The problem is, just a wee bit too much fertilizer will kill your plants. The pH of the soil has the ability to remove chemicals from access by your plants. The temperatures do the same thing. Some of the chemicals necessary if added in too large amounts or if deficient will also cause some chemicals to not be accessible to plants. If the soil has no life; mycorrhize, bacterias necessary for plant roots to up take chemicals that are in the soil, that will hamper plants as well to be able to access and up take necessary chemicals to make food/energy with which to make more vegetative growth or reproductive growth.
In addition, one of the most important concepts to produce reproductive growth versus vegetative growth is the proportions of chemicals added. If for instance you add a fertilizer with a formulation of 7-6-4, you will promote vegetative growth. If you add a fertilizer with a formulation of 10-12-14 you will enhance reproductive growth and the propensity to be able to get larger fruits. Too much Nitrogen in relation to the Phosphorus and Potassium percentages will enhance vegetative or leafy growth. Reproductive growth will suffer.
Great for salad greens, kale, rhubarb but if you want reproductive growth; raspberries, tomatoes, squash, cucumbers you need to watch carefully that the Nitrogen is less than the other two macro chemicals.
These are chemicals not nutrients or food. That is why I try to not use the word 'nutrient' as people think of nutrients in their own terms of food/fuel. Too much food for us just makes us fat not dead. For plants, there is an optimum amount of chemistry; a little too much of one chemical causes symptoms of excess, too little causes symptoms of deficiency and just a little too much of a few chemicals will most certainly kill plants.
Fertilizing is an important science we humans have to learn and understand. Stable ecosystems where we humans have not had the chance to disturb are so finely tuned there is just enough for the health of plants until something is allowed to die to be added back into the ecosystem for use by that ecosystem.
We humans go in and remove the biomass of these ecosystems and that soil will be deplete for the needs of plants we want to grow or for new plants in that ecosystem. The rainforest for instance. We burn and cut down the rainforest and that land is worthless for crops. Maybe the first year there might be enough chemistry to grow one crop but that is it. Most people have heard this but this is true every time we remove the forests and expect yummy soil to grow our crops. We humans are actually mining the chemistry of that soil by removing the biomass.
We have to learn how to add chemicals back into the soil so that we are able to grow crops for ourselves to eat. If we do not understand the chemistry in our soils and do not add fertilizer because some article says that fertilizer is bad we have misunderstood the entire biological/botanical/mineral parts of the system and we will not be able to grow anything with success...
Soils are an amazing science. The only way that has been found to improve our soil texture/structure/organics to enhance botanical growth is by the addition of already decomposed organic matter. To decompose organic matter organisms that are in charge of decomposition need lots of nitrogen. Once they have done their job by decomposing something newly dead, only then is that organic matter now decomposed available for use by micro and macro organisms in the soil to eat for energy. While organic material is being decomposed all the other organisms go to sleep, dormancy or die. Once the decomposers have decomposed organic matter there is then 'food' for the micro and macro organisms to wake up and reproduce. Plants need to have these micro and macro organisms to uptake chemicals they need for photosynthesis (to make food and seed). The decomposed organic matter is critical as well to improve the tilth of the soil for air, organisms and plant roots to thrive.
What is it that you are wanting to accomplish? Large tomatoes, more tomatoes, longer lasting tomato plants? What have you been using for management skills? What hasn't been working?
Hope this helps generate more questions...ask away!