I've got an "XS" Pokémon. Is it special?

Solution 1:

There is nothing particularly special about an extra small pokémon. Typically, smaller pokémon appear to start with lower CP values, but that is about it.

That said, there may be further medals that relate to the weight of a pokémon. For example, there is the Youngster medal.

Solution 2:

A Pokémon with an XS tag, for either weight or height, is a sign that this Pokémon has lower than average values for those stats. As was explored in this question, it appears that higher weight leads to higher health for two Pokémon of the same CP value. From this, one can conclude that a Pokémon with XL tags for height and weight is generally going to be a better choice than the same Pokémon with XS tags.

I've read in a few places that these tags also influence your Pokémon's moveset, although I haven't seen any evidence to back this up.