1. The Federation spans 8,000 light years. Simply put, it is at least twice the size of the Romulan Star Empire or the size of the Klingon and Romulan Empires put together!
The Federation admits new members who (supposedly) wish to enter the Federation. Entire regions would contain multiple intelligent warp cultures with no interest in the Federation. So there could be big gaps separating Federation members. The two Empires would observe no such admissions policy, pulling new "members'' into their Empires through conquest and colonization.
So the two Empires could in fact each be physically smaller in area, more compact, while at the same time having just as many star systems as the Federation. Equal amounts of resources.
The Klingons and the Cardassians share a border, on the map you (abundantly) displayed this would mean the Klingon Empire has to extend either over or under the Federation in order to accomplish this. So the Klingon Empire is bigger than this 2D map shows.
You can't have as many star systems as the Federation if you don't encompass as much area. And the blue section includes all the planets that are part of the Federation and it is clear to anyone who possesses the ability to read that the Federation includes more planets than the Klingon or Romulan Empires.