The next spinoff to tackle: DSC. In Burnham's War during the first season, Klingons make immense territorial gains, yet we never hear of high casualties. The initial tally is 8,000 or so Starfleet fallen; this goes up to only 10,000 by the Harry Mudd time loop adventure. No civilian losses are mentioned. Then comes the side trip to the Mirror Universe for nine months, after which the territorial losses are reality but Admiral Cornwell lists 11,000 dead civilians when Klingons destroy the entire atmosphere of Kelfour VI; three suicide raids against starship harbors; and tree outpost raids on border colonies that leave some children alive.
Now, Cornwell is showcasing the diversity of Klingon attacks and the underlying fact that the enemy is not united on a strategy. But it would be unlikely for her to omit even greater losses from her list of examples. Her mention of the 11,000 dead is thus telling both in terms of what counts as shocking, and what counts as loss of total planetary population. Too bad we never learn what sort of a place Kelfour VI was - an outpost, a colony, a homeworld to Kelfourites? Subsequently, Starbase 1 is lost to a Klingon raid, and the 80,000 people aboard are supposedly dead. No higher numbers for civilian or Starfleet losses are ever quoted either for specific attacks or for an overall tally.
Either the part of the UFP facing the Klingons was unusually sparsely populated or then not. In the latter case, smallish colonies are indirectly established overall; in the former, locally.
FWIW, Marcos XII was somewhat close to Triacus which had dealings with Epsilon Indi, basically putting the action in the areas shown conquered by Klingons. We have no idea where Deneva might be, save for the Star Charts conjecture that can sort of be seen on screen if one squints hard enough at those war maps and pretends a lot.
Timo Saloniemi