They don't have to be like bacteria to have a massive population. You do know that a sextillion is, say, a million times more than a quadrillion right? They could be the most unfortunate species in the galaxy, suffer all the catastrophies possible, and could easily reach the quadrillion range.
Actually, they DO have to be like bacteria to have a massive population - well fed bacteria, that is.
And no, they couldn't be the most unfortunate species in the galaxy and reach the quadrillion range.
All it takes is for the average romulan couple to have 2 children - or less.
That's it for population increase.
PS - Easily? Within the confines of the trekverse (as presented) or the real world - well, that's laughable!
Not these diseases.
But many others - with similar effects, demographically.
Many of them mentioned in trek canon.
EmperorTiberius - in trek, populations suffer from the same "gremlins" human populations suffered throughout history - regardless of 'colonies' or any other hand-waving you would care to use.
Again, many - most - of them mentioned in trek canon.
You think it not realistic?
Well, fantasy science replicators or transporters are not realistic, either.
You can't pick and choose what you consider 'realistic' and what not (well, you could, but it would make your posts worthless).
Your estimates of 40k are laughable.
During history, many colonists/lineages/populations suffered from a far worse fate - number of descendants: 0.
US has had a growth rate between .9 and 2% for the past 100 years. Are you saying people in US grow unrealisticly like bacteria? Are you saying that advanced Vulcan technology in hands of agressive, expansionist Romulans woudn't be able to match this type of growth?
Take a look at demographic increase in developed countries (including USA).
You'll find it goes backwards, stagnates, or barely advances these days (at most).
So no, the populations of rich countries definitely do not breed like bacteria.
Of course, in the trekverse, the conditions - dangerous diseases/wars/lacking food/etc - presented are more like those at the beginning of the 19th century.
PS - For the USA:
One - of many - relevant information: "The total fertility rate
in the United States estimated for 2011 is 1.89 children per woman, which is below the replacement fertility rate
of approximately 2.1"
Next time, actually do a google search yourself.