In my experience of growing a wide variety of fruit, the general rule is that smaller ones are sweeter, because the sugar is more concentrated in the fruit. Up until recently, commercial varieties were often selected based on yield with little regard to flavour, and this meant bigger fruits with less flavour. Nowadays, there has been a revival of flavour over yield, thankfully.
An exception to the rule is the small under-developed and unripe fruit. Pretty obvious I suppose, but needs saying. Tiny unripe fruits will not be sweeter than bigger, ripe ones!