I do realize I'm pretty late to this thread, but figured I had pertinent information.
I'm in Fire Academy right now, and depending on your build, you are most likely better off with some more weight on your frame. I'm 6'2" ~195 (~9%), and on CPATs I absolutely DESTROY a guy in my class that is arguably a much better athlete, but weighs only 155lb. How much better? He can run a low 15min 5K (as opposed to my ~21min), do more pushups, pullups, situps.. He just doesnt respond as well to putting the same weight on his back as I do. Typical times on mock CPATs would be his 3:13 to my 2:18, or his 5:17 to my 4:38. This is a theme this year, with a number of people of lighter weight having significant trouble once you put on bunker gear and an SCBA.. let alone try and drag a 175lb dummy.
That said, a very effective way to gain weight is GOMAD. Despite being a hard gainer, I'm up 9lb in a month (though I'm sure some of that is water). I'm planning on trying to gain another 10-15, then work on maintaining that and getting as quick as possible. It is definitely possible to be TOO big.. not only from the perspective of unnecessary bulk slowing you down, but it getting difficult to fit through obstacles on the fireground: literally a potential matter of life & death.
Dont forget about met-con though.. the CPATs are intense enough, and long enough, to give someone without good conditioning serious trouble.
Good luck with it.. I just cant put into words how much I enjoy what I do.