Probably the decisive difference between minor and major powers isn't simply fleet size or number of possessions, but the degree to which the military and industry can approach the cutting edge. Cardassians were shown as hopelessly backward in military technology in TNG, incapable of challenging Starfleet ships even with major numerical superiority. In that, they joined Talarians and no doubt a great number of other two-bit villain cultures that nevertheless had warred against a fringe of the mighty Federation, and lost.
What it would take to become a Romulan- or Klingon-level threat to the Federation is sufficiently modern weaponry, and sufficiently modern technology to mass-produce it. During DS9, Cardassians finally seemed to be getting there - but nevertheless, their ships still were easy prey to Klingon or UFP counterparts, right until the Dominion came and offered help. It's only around this time that Cardassian capital ship phasers gain in brightness and start to make serious dents in enemy ships...
So I'd say second-rate power all the way, but that's a compliment: most opponents are actually third-rate powers, lacking farther behind in technology and also having markedly smaller possessions. Conversely, there are plenty of "negative-rate powers" that can challenge and defeat the Federation despite not really having much in the way of planetary possessions, as they operate such divinely superior technology - say, the Thasians, Organians, Metrons and so forth.
Timo Saloniemi