Yeah, I always figured the Klingon empire was stronger than it looked. Probably after the Praxis situation and the steps towards uneasy peace at Khitomer, the empire was probably provided with all sorts of financial assistance which helped them to grow and prosper -- not unlike Japan following WWII. She might look somewhat meek, but beneath the surface she might be equal to the Federation again by the time of TNG, possibly even slightly stronger
. As has been mentioned already in this thread, in one alternative timeline we got to see, they were able to beat down Starfleet with very little trouble at all; while in another
alternative future they had managed to conquer the Romulan empire and had become antagonists to Starfleet again. Even in our own 'official' time-frame we saw their military might come out during the Dominion conflict.
The strength of the Romulans is harder to guage due to us seeing really very little and of their general secrecy. It's hard to get a big picture of how strong they are, but my impression has always been that the answer is "not very". Even in Nemesis
, where the story is effectively centered upon them, I kept feeling that there's really very little evidence of them having a massive battle fleet. They've got some impressive ships, and are very good at pumping themselves up as being Big Playas on the galactic stage. But I'd wager that a lot of it is bluster, hiding what is actually
a very paltry military force compared to the Klingons and certainly the Federation.