Why do small things taste better than big things?

A few factors are in play here (and these probably aren't all of them). I'll stick mainly to fruits (in a rough botanical sense) but some of this will generalise more widely.

Many commercial varieties have been bred for yield (as expressed by weight) and appearance. This is likely to have come at the expense of other variables, such as flavour. This leaves room in the market for tastier varieties, even if smaller or a less attractive colour or shape.

For a given variety of some fruits, water uptake can make a big difference. I have wild blackberries at the bottom of my garden, and don't water them. In a wet year they're juicy but not very tasty. In a dry year, they've got plenty of flavour, but they're hard - good for cooking with apples but not for just eating. I'll try to track down an article I read on arid agriculture - careful minimal irrigation could produce a premium flavour but a low yield. In the mean time here's an article coincidentally published today saying the same for herbs.

Some fruits stop growing long before they ripen, while others don't. This, combined with the timing of water availability and sunshine when needed for ripening, will affect the concentration of flavour compounds produced during ripening.