[My knowledge of High Medieval period is based mostly on listening to TTC's audiobook lecture series about it (highly recommended), not on Wikipedia.]
A few random thoughts:
I don't have one big mathematically pretty theory here. (Short-term) Malthusian theory seems to be widely accepted because it's mathematically pretty, and something like it works on many animal populations. It's not obvious from either theoretical or empirical point of view it makes sense for humans, at least after they stopped hunting and gathering own food.
Actual food production must cover actual population no matter if Malthusian hypothesis is true or not - if food production was too low, people would die; if food production was too high, there would be nobody to eat it (unless people do something funny with their food instead of eating it). So that's the wrong thing to look at. Lower population levels must automatically mean lower land area used, or lower productivity/intensity of agriculture. And we can guess people would use the "best" lands, and leave "worse" lands unused.
I think failure of Medieval Europe to rebound back to Roman population levels is much bigger problem for Malthusian theory than Tuscany's. Small highly urbanized regions like Tuscany might have simply imported food from other regions. Such explanations won't work for an entire continent.
TTC lectures present as well-established fact that quality of life increased a lot in High Medieval period, while population roughly doubled. The lecturer finds it quite puzzling, as it conflicts with Malthusian theory.
Wikipedia says that during the entire Medieval period, "land was plentiful while labour to clear and work the land was scarce". This is highly un-Malthusian scenario. Malthusian theory strongly predicts having large excess of labour.
Obviously there must be something that limits fertility. Malthusian theory says it must be potential food production, and that every land that can produce enough food to support people farming it must be used. If the mechanism is something else, we might have highly non-Malthusian (non-poor) equilibria. Wars, and failure to organize expansion to new lands (what in agricultural societies might require significant up-front investment, it definitely did in Medieval Europe, settlers on new lands were freed from usual taxes for many years) are a few obvious mechanisms.
Wikipedia says position of peasants in Western Europe which had high labour-to-land ratios was much better than in Eastern Europe with high land-to-labour ratios. Malthusian theory predicts otherwise. I can see no differences in agricultural technology, or land productivity, or anything else to explain this away.
Increase in wages after Black Death can as easily be interpreted as inflation, not as increase in gdp per capita, as prices seem to have increased correspondingly. It's not really surprising, as supply of metal money stayed constant, so supply of money per capita increased proportionally to loss of population.
Wikipedia says that Black Death had negative effects on the economy:
Economic historians like Fernand Braudel have concluded that Black Death exacerbated a recession in the European economy that had been under way since the beginning of the century.
We have really crappy data about economic history.
One issue to keep in mind is measuring wealth in calories and in other terms. People interested in malthusian theories focus on calories, while those interested in the advance of civilization may measure it in, for example, cloth. The malthusian theory predicts that the proportion of salary that goes to food should creep up in peacetime. But the advance of cities in the high medieval period made other products cheaper. When Braudel talks about economic recession, he's probably talking about abandonment of cities, which is not necessarily relevant to the ma...
This is an attempt to list all of the possible ways in which humanity may avoid scenarios where the average standard of living is close to subsistence, in response to Robin Hanson's recent series of posts on Overcoming Bias, where he argues that such an outcome is likely in the long run.
I'll start with six, some suggested by myself, and others collected from comments on Overcoming Bias and Robin's own posts. If anyone provides additional ideas, I'll add them to the list.
(I have a more general point here, BTW, which is that predicting the far future is very difficult. Before thinking that some outcome is inevitable or highly likely, it's a good idea to repeatedly ask oneself "This is all the ways that I can think of why it may fail to come true. Am I sure that all of them have low probability and that I'm not missing anything?" There may be some scenario with a non-negligible probability that your brain simply overlooked when you first asked it.)
Singleton
A world government or superpower imposes a population control policy over the whole world.
Strong Security
Strong defensive technologies and doctrines (such as Mutually Assured Destruction) allow nations, communities, and maybe tribes and families to unilaterally limit their populations within their own borders, while holding off hordes of would-be invaders and immigrants.
Non-Human Capital
Maximizing the wealth and power of a nation requires an optimal mix of human and non-human capital. Nations that fail to adopt population controls find their relative wealth and power fade over time as their mixes deviate from the optimum (i.e., they find themselves spending too much resources on raising humans, and not enough on building machines), and either move to correct this or are taken over by stronger powers. (I believe that historically this was the reason China adopted its one-child policy.)
Unlimited Growth
We don't completely understand the laws of physics, nor the nature of value. There turns out to be some way for economic growth to continue without limit. (Robin himself once wrote "I know of no law limiting economic value per atom" but apparently changed his mind later.)
Selfish Memes
Memes that manage to divert people's resources away from biological reproduction and towards memetic reproduction will have an advantage over memes that don't. On the other hand, genes that manage to block such memes will have an advantage over genes that don't. Memes manage to keep the upper hand in this struggle (or periodically regain the upper hand).
Disease, Warfare, Natural Disasters, Aliens, Keeper of the Simulation
One or more of these come along regularly to keep the human population in check and per capita incomes above subsistence.