For years I watched and read Star Trek and always imagined from what I've seen that semi-military Starfleet is about equal in military power to Romulans, and slightly weaker than the on-perpetual-war-footing Klingons. They're obviously slightly stronger than Cardassians. Balance of power is kept by careful diplomacy, aliances, etc.
Then, when I started visiting Trek forums, I noticed fans talk about "Super Power" Federation as the only true power in the Alpha Quadrant (most often it's compared to the U.S.). I also noticed that people think Romulans are supposed to be a small empire like the Klingons; and Cardassians are considered insignificant ants that Starfleet can brush aside at any time.
Where is this fans' impression coming from?