In the end I think this comes down to us simply not having enough information.You ask why we should assume the colony is small, I ask why we should assume it's large. Neither of us has compelling evidence. I'd be a lot more disturbed by what Sisko did if it didn't essentially "balance the scales", and I'll admit I grow increasingly less disturbed by it the more I see forum posters in general accusing Sisko of "poisoning a planet" while glossing over the particulars. That said, we're of course talking about an episode of a television show rather than something that actually happened. That limits my threshold of investment.