Are standard metrics of academic output skewed by the relative popularity of a field?
The comments are already spot on, but let me elaborate a bit.

Comparing h-indices (or any other "hard" metric) is already dangerous in narrow fields and downright foolish if used for comparisons among different fields. This is not only because some fields are larger than others, but also because:


Differences in publication standards. In applied CS we write lots of papers, in many natural sciences, much fewer papers get written per researcher and time period. Arguably, this is because many empirical fields require the setup and analysis of lengthy experiments, something that is not typically (but not never) done in CS.
Difference in co-author ethics. Just check around here on this stack exchange, and you will see that standards for co-authorship are not at all uniform in all fields. Clearly, fields with more loose co-authorship standards also expect researchers to be part of more paper projects, hence leading to higher h-indices on average.
Differences in citation standards. In some fields, papers traditionally have 10 or less citations. In others, multiple dozen references are considered an informal minimum, again leading to higher average h-indices.


(and this is even without going into how easy h-indices are to manipulate if you are willing to - keyword "citation rings")