Somewhere in life I've heard a children's/novelty song about a one ton tomato; eventually (if I remember correctly) it ends up as a similar quantity of ketchup.
Nearly half-ton pumpkins show up pretty regularly at the big agricultural fairs every fall, but tomatoes aren't in that league. But, the difference between an ancestral tomato (small berries) and a multi-pound beefsteak is nothing to sneeze at. Domestication has made great strides.
A paper last month in Nature Genetics laid out part of this process. Interestingly, there are two different developmental processes that have been utilized to enlarge tomatoes. A tomato fruit is composed of multiple subunits, the carpels. One change has increased the number of cells per carpel by tinkering with the cell cycle -- a much more delicious change than what a similar process will yield in a person. The new work details the genetic change which increased the number of carpels.
Of course, of interest is how universal these mechanisms are. Most domestic fruits are greatly enlarged over their wild counterparts -- though perhaps raspberries show very little enlargement & blueberries it is a small multiple. On the other end are those monster curcurbits at the fair and their watermelon cousins.
But getting back to the title. Now the question is whether these mechanisms have reached their biological maximum or simply what a few mutations can do (there are also practical considerations, such as the stem strength required to support larger tomatoes). Or, can we use this new knowledge to bring up the laggards -- or figure out why there are no fist-sized raspberries or basketball-like blueberries? A strawberry the size of my dog? Of course, purely economic forces might lead to the fruits commanding the most money per unit weight -- perhaps pomegranates will have an order of magnitude more seeds! Healthy for you -- so long as you watch where you eat them.