I have half a dozen Noi Na (sorry, I do not know the name in English, maybe there is not one) trees in my garden, at least two are bird sown. They are all now coming into fruit, but, approx. half the fruit on each tree is much smaller than the rest. Are these fruits likely to develop to full size, or should I cull them to give the bigger ones a better chance ?

Noi na is "custard apple".  You're blessed if birds are gifting you with this incredible fruit. 

 

I love noi na but I haven't grown it, so I'm not sure if it normally matures and ripens simultaneously, or staggered over a period of time.  I think the latter is true. Growers would know. 

 

Provide good water management and fertility and wait to see if the smaller fruit matures later. 

 

If the smaller fruit is located on one side of the tree, as opposed to being scattered throughout the canopy among the larger fruit, then there may be issues with growing conditions and root development, or tree structure that is impairing uptake of water and nutrients to the affected side. 

 

Post photos for better feedback.

 

As with most fruits, commercial trees are hybrids, and probably grafted. God bless your seedlings, but they have reverted back to whence they came, maybe you can increase fruit size a little with irrigation and fertiliser.