What makes small tea leaves better than big tea leaves?
Before I answer this question, the focus on small leaves and young buds is mainly relevant for green and white teas. 

This category of teas are very lightly processed and are therefore very 'raw'. Using very large leaves would result in a more bitter brew. Therefore small tips and buds are the best and result in more delicate brews. 

The smaller leaves are also picked in early Spring. As the temperatures are low, they grow slower and are thus richer in flavor.

These Spring buds have less supply, while there's less demand. As you can imagine, that results in higher prices.