I believe you are mistaken about that. The fruit will be smaller on a mature bonsai for the same reason that the leaves are smaller, and the flowers are smaller too. The fruit is not an exception to this. Seeds from the fruit will not produce small trees of course, because as you said they are genetically the same, but the fruit will be smaller on the bonsai.
I would be very very interested in seeing an example of this.
This is incorrect, To be specific: Neither Fruit nor Flowers reduce in size. The reason you often see bonsai with smaller sized flowers is that people use cultivars or sports that have smaller blooms in the first place. Because they are much more in scale with their intended design.
If it were possible to reduce them you would see bonsai being made out of some species that have suitable woody interesting growth habits and huge flowers..
['To be reduced later, Large flowers? No problem, Pick any species you want...']
But you don't in fact see that at all because there is nothing we do wrt the enviroment of bonsai that changes the genetic factors which affect fruit/flower size. Try to change a full size rose into a mini rose and see.
As Gnome mentioned Malus are very popular for those who desire fruiting bonsai, They have alot of appeal in all four seasons IMO.
Btw- femlow you mention this:
When you have a young tree, the leaves are larger, so you cut the first flush and the second flush comes back smaller
The aim of defoiliation in bonsai is not
in fact leaf reduction, It is to promote ramification. Leaf reduction is a natural beneficial side effect of this [Given good lighting] but not the primary aim.
See https://www.evergreengardenworks.com/leafsize.htm for a few notes about leaf reduction for bonsai. It is the last thing on the list, after all of the other training is achieved.