to NGA
Somehow the conditions are different: sunnier or shadier, dryer or wetter, better soil.... Its impossible to get exactly the same conditions but even that is no assurance they will all grow equally well. Annuals (which squash are) have only one thing in mind - ensuring that there will be a next generation. The ones that are smaller but producing small fruits have decided its time to make sure that happens. The bigger ones aren't feeling that stress to reproduce quite yet (but hopefully they will eventually) and so are growing bigger before fruiting.
Hope this helps.
Daisy
