One thing that troubles me slightly with this conversation is that I think there is a presumption that people will naturally choose to read and write Wikipedia in their native language, but that isn't necessarily so.
Anecdotally it seems many people read English Wikipedia because precisely it is larger and more comprehensive (obviously they must have a reasonable ability to read English). And I would imagine that this is true too for, say, Catalan, where one might imagine they would also know Spanish and might well turn to the larger Spanish Wikipedia most of the time. Generally speaking, speakers of "small" languages (meaning small populations of native speakers) are likely to speak one or more "larger" language and therefore may preferentially read Wikipedia in those "larger" languages in order to have a broader and deeper array of content. As I don't speak any "small" languages myself, I do not know if those Wikipedias tend to cover the more general topics or whether there is a greater focus on local content unlikely to be covered in "larger" Wikipedias - does anyone know?
If this is true about reading Wikipedia, then it seems likely to flow over into writing Wikipedia as well. Writing for the "larger" language has the benefit of bringing information to more people. So, here, motivation for editing comes into play. I suspect people who write for the "small" language Wikipedias probably have a motivation to keep their language alive, whereas this is unlikely to be a consideration for the large languages. But OTOH if you write for Wikipedia because you are passionate about sharing your knowledge of a topic area (e.g. Pokemon, football, cactus), then it seems that you would write in the Wikipedia with the largest content base on that topic (within your linguistic abilities) as you would have more to build on and a larger community of other editors to work with. Of course, working with others on Wikipedia isn't always easy, and perhaps that might be a factor that might drive an editor to write in a "smaller" language Wikipedia (which might be more work, but with less conflict).
What I don't know is whether any of these issues are microscopic or macroscopic. If they are macroscopic, then they have to be factored into the model of "how a Wikipedia should develop".
My personal view is that the "extremely small" language Wikipedias are unlikely to achieve a broad coverage of general topics because they are unlikely to find a large enough editor community. I think they will underperform whatever level of development they might theoretically be capable of. My rationale is that we know that Wikipedia is written predominantly by people with higher than average levels of education, which almost certainly means you have had to learn one or more larger languages to do this, thus opening up the ability to work with other Wikipedis, thus siphoning off some proportion of the editor base to boost the development of larger Wikipedias at the expense of their native-language Wikipedia. I think it is more realistic to focus on more local content in small language Wikipedias and leave the more general content to the larger Wikipedias.
Note, this is all written on the assumption of not using machine translation. Clearly with machine translation, there is far greater potential for content in languages for which there are machine translation tools. But again, machine translation is less likely to be available for the "very small" languages, so even in that scenario, I think the smaller language Wikipedias will miss out on the content.
Kerry