The Cardassians did previously manage to fight the Federation to a standstill, and achieve possession of a portion of the territory that both the Union and the Federation coveted.
As of the start of DS9, where the only planet known to change ownership in the aftermath of the "Chain of Command" debacle is Bajor, the very Cardassian home system suddenly borders on neutral space in the direction of the UFP.
...Well, space filled with cultures sympathetic to the Union, or at least with cultures hostile to the Federation, such as the Kressari, Klaestron or Xepolites, plus Miradorn, Tzenkethi and so forth. But still space not belonging to the Union. That doesn't sound like the spoils of a war fought to a standstill - but more like a home system left to its owners out of pity when everything else has been stripped away from them.
It's just that when the UFP wins wars hands down, it doesn't annex territory...