Just off the top of my head, I'm trying to come up with a larger single nation in area (OTL) that was conquered militarily than Mexico in 1848 and larger nation that was conquered with control kept after conquest than the CSA in 1865. (I don't have exact measurements for the square mileage for either though)
Any ideas? I *guess* China in the Opium wars for the first, but I have no good comparison of the land areas. And while Australia would also be in the running for the second, the colonization of Australia doesn't represent *one* military conflict in the classic sense.
There are certainly larger examples for the second in ATL ranging from Canada in TL-191 to a larger US in something like the Draka or Nazi Victorious TLs..
(I'm not restricting this to pre-1900, it just seems to fit better in this forum)