system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] Are there any other races in the Klingon and Romulan empires? If not, how is this possible given the large scale of both empires?

The Federation has at least well over 100 member worlds [sources vary, but it's at least 150 worlds in the TNG era], and we commonly see many different species from all across the Federation.  

The Romulan and Klingon empires are smaller than the Federation, with maps of different sources varying to a great degree.  However, both empires are within an order of magnitude of the size of the Federation.  Where are the other species in those empires?  Why are they never shown?  

And if there are no other species within the Romulan and Klingon empires, how is this possible given their relative sizes?
gpt
The Remans are part of the Romulan Empire.  Kriosans are a part of the Klingon Empire.  The reason you don't see a lot of the "client races" of those Empires is because they are not allowed to serve on their starships alongside Romulans or Klingons.