Some fruits such as pumpkins can grow to be 100 lbs. Under different conditions, the same variety of pumpkin can produce a 15 lb. fruit. Both plants are healthy, and look the same except for their size difference. If I took two red apple trees and planted them in extreme opposite conditions, the fruit size would be nearly the same. The fruit quantity would be lowered in the tree grown under poor conditions. Why do fruits have such a variable range of acceptable sizes?
Pumpkins, squashes in general, grow on vines, while apples grow on trees. Vines are fast growing and trees are not. Zucchini can be quite large; cucumbers, too. Pears, plums, peaches and other tree fruits do have a reduced variation in fruit size. While I do not know the answer to your question, my background in plant biology tells me that this is an important part of the reason. For example, a watermelon or pumpkin rests on the earth, but a tree fruit hangs. If that fruit is too heavy it falls from the ground before it is ripe - before the seeds are fully differentiated that is. So, while "evolution" might be an answer, it is important to discuss what aspects are at play.
I think it has to do with vine vs tree, and how the fruit is attached and hangs/lays.
Bigger pumpkins have more seeds than smaller pumpkins. When they get ideal nutrition the plant tries to make as many seeds as it can to take advantage of the windfall. Many of these vines with big fruit potential grow in compost and rotting material. So they are more opportunistic a pumpkin seed "in the wild" could land in a barren area or in a heap of rotting material-- if it hits the jackpot it meeds to quickly use those nutrients to make more seeds.
PLants with more consistent fruit sizes don't respond as well to "jack pot" levels of rotting material-- they like more consistent nourishment. I think this is another difference.
Also one large pumpkin is a more efficient way to enclose seeds than lots of little pumpkins.
I suspect the answer to this, is that in looking at agricultural fruits you are looking at plants that have been the subject of artificial selection, and the tree vs vine thing is a practical limit, but not the driving factor... for instance a jack fruit grows on tree and is watermelon sized...
I think if you look at something like a pumpkin, you will likely see that pumpkins have 3 traits:
- Indeterminate growth ... they will produce as many fruit as the season will let them.
- Individual fruit will try to complete growth and ripen before winter
- Vines with fewer fruit can devote more resources per fruit, and therefor larger fruit.
Even in apples; if all of the blooms go to fruit the apples will be smaller (commercially apples are thinned to make market acceptable sized fruit): http://homeorchard.ucdavis.edu/8047.pdf
So I think the real issue is less tree vs vine, and more about growth patterns... it may be possible to have a watermelon sized apple, but the tree will have to be sturdy enough to support the fruit: so it isn't a non-issue, but support is a limiting factor.