The Romulans could have also given up some or a lot of space to the Federation. Maybe, like during the Dominion War (when the Dominion was making concessions to gain a planet that had some of the resources to make the White), the Federation gave up cloaking technology to gain scarce resources on a planet now on their side of the Neutral Zone.
In a good compromise, or Treaty (a treaty to end or stop a war where there is no victor as of yet), neither side is completely happy. The Federation had to expend resources and talent on developing better sensors, both sides lost a chunk of space, and the Romulans had to pour resources into new cloaking technology. The Tal'Shiar and Section 31 or Starfleet Intelligence had to work over time to get the new sensor/cloaking information back to their governments.