What makes small tea leaves better than big tea leaves? According to wikipedia, the smaller the leaf (especially the buds) the more valuable it is.




  Tea leaves of different sizes just after plucking. Small leaves are
  more valuable than big ones.


From what I gather, the biggest quality pickings are the buds. Also, tea quality grading takes into account how many leaves were plucked alongside the young leaves.


  Pekoe tea grades are classified into various qualities, each
  determined by how many of the adjacent young leaves (two, one, or
  none) were picked along with the leaf buds. Top-quality pekoe grades
  consist of only the leaf buds, which are picked using the balls of the
  fingertips.


Why does the number of leaves that were picked alongside the bud matter?

Also, it seems like the lower the leaves, the lower the quality. Maybe it's about sun exposure, or is it simply age?
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.