How could the Klingon and/or Romulan empires compete with the Federation?

[RedshirtP4C-970]

The Federation, in TNG's time, is made up of a vast number of highly industrial worlds that are not colonies of Earth - the Vulcans, the Andorians, etc. Whereas both the Romulan and Klingon Empires only seem to be made up of Romulans and Klingons. As such, given the vast resources and populations that are part of the Federation, how can those two Empires compete and at times (and episodes) defeat the Federation?

 

[Elias Vaughn]

They're empires. You can probably assume a fair amount of behind the scenes slave labor, like the Remans.

They are also typically more ruthless. It's pretty easy to compete with an entity that doesn't actually make use of all its power and force.

 

[Mister Spock]

Ronald D. Moore outlined how the Klingon Empire worked in a memo to Michael Piller before he started writing Sins of the Father.

Unlike the United Federation of Planets, the Klingon Empire is not an amalgam of several different star systems brought together by common purpose and values. The Klingon Empire sprang from a single, relatively poor planet in a modest star system. The worlds that now make up the Empire were either subjugated in a not-so-distant past or were annexed at the point of a sword. The Empire is efficiently managed and extremely well run. No star system has ever broken away from Klingon rule in over two centuries of steady conquest. This is not to say that the member worlds of the Klingon Empire are straining at the bit to break away from despotic rule. Quite the contrary, the member worlds of the Empire have learned the many advantages and benefits of their association with the Klingons and few would choose to leave, even if given the option.