What is the difference between small vs large loose leaf tea?
Smaller leaves typically  but not always would mean younger leaves and is associated with higher quality. However this is not always the case and especially by this particular tea I don't know how true this is. 

This tea is supposed to be made by growing tea in Taiwan without pesticides. Having a particular bug chew on the leaves causing the plant to produce certain chemicals that give this tea its flavor. 

Additionally it's supposed to only use the top two leaves. 

All these things together could make this potentially a pretty expensive tea. So some manufacturers take various shortcuts life chopping the leaves so it looks like a smaller tip leaf but actually being a older leaf. 
Also being grown in Taiwan which has somewhat unstable weather and being dependent on those bugs makes this a very seasonal product that may be great some years and bland the next.  

https://en.m.wikipedia.org/wiki/Tea_leaf_grading

https://en.m.wikipedia.org/wiki/Dongfang_Meiren