The Romulan Star Empire was massive. Depending on where you get your info from, it may have even rivalled the Federation in terms of size and power.
Romulus being destroyed was a major defeat, but that did not detroy the Romulan Empire nor every Romulan out there. They are wounded but not destroyed.
Going back to TOS-era concepts, the Klingons were supposed to be the Soviets, and the Romulans the Chinese. At the time, we had cut off all diplomatic ties with China, and this was what was mirrored in the ideas of not being able to speak with Romulans, not even knowing what they looked like.
So from that, if you think of the Federation as the United States, then the Romulan Empire is like China.