What rough estimates there are suggest that transgender people are overrepresented in the military. Perhaps dramatically so.
The most prominent of these estimates comes from the Williams Institute, an LGBT-focused think tank based out of UCLA. In a report from 2014, authors Gary Gates and Jody Herman estimate that approximately 15,500 transgender men and women are serving and that an additional 134,300 trans Americans are veterans. Given a national population of 700,000 (another rough estimate), this suggests that over 1-in-5 (or 21.4%) of all openly transgender Americans are in the military or have served at one point.
As we noted previously, Transgender Americans, in other words, are estimated to be twice as likely to join the military.