Nero and his crew probably those people who in the face of a natural disaster (Hurricane Sandy/Katrina) know the warning but don't think to move their families away from home.
Given that Nero had his own massive ship, capable of transporting tens of thousands off of Romulas, including the families of Nero's crew (and his wife), and he didn't, is one of the biggest glaring plot holes of the last movie.
The Romulan Empire is so centralized (see: Nemesis, where one room full of people are killed and Shinzon effortlessly takes over) that a collapse is very much a possibility.
Shinzon had the support of the Romulan military prior to assassinating the politicians.
A generation being born every 30 years - in 2000 years, at 102% growth rate per generation, after 67 generations, their number would be 683400.
Going beyond modest growth.
If Romulans arrived on Romulas 2000 years ago and produced another generation on average each 50 years, that would mean that 40 generations have passed since first arrival.
And if they gradually colonized/invaded other surrounding worlds.
If we started with 10,000 people, and each couple in each generation produced an average of 3 children who in turn reproduced, the population of the Empire could be up to 110 billion people.
That about 16 planets with current Earth population, or one planet (Romulas) with Earth population, and 1,000 planets with a hundred million population.
Four children per generation (50 years) gives you 10,995,116,277,000,000 people.
And if they had five children (like my parents did) per couple, that would give them enough people after 2,000 years to populate 1.8 million planets with a current Earth population.
Here's the population calculator I used (kind of cool). http://wardricker.com/timegrowth.php