I had been wondering about this apparent contradiction for a while and it seems my faulty understanding of the agricultural revolution led me to all kinds of wrong conclusions. Evidence is accumulating that Flannerty's broad spectrum revolution hypothesis accurately predicted that an increase of dietary breadth due to the culling of large game populations preceded the neolithic. Levantine hunter-gatherers began to exploit various wild cereal sources as well as unlikely animal sources like rodents, frogs, shellfish that were extremely difficult to hunt. As the population increased, the adoption of agriculture became a matter of life or death.
European Mesolithic societies did not yet face such dire circumstances and likely did not anticipate the future developmental potentional of primitive agriculture. In summary, European hunters were well-fed but stagnant while agriculturists were malnourished and dynamic. Hence West Asia became a population-pump and Europe did not. That's also why it makes little sense to look for an European origin of those Neolithic haplogroups.
I agree with much of this, but I also think it very much depends on which farmers you're discussing. It very much depends on the area and the time period. Of course, if the rains fail, if there's a flood, if there's an infestation of locusts, if they've just worn out the soil because they don't yet understand how to replenish it, the farmers are going to suffer from malnutrition. When things are going well, they're not, particularly the groups who also incorporated some fishing and hunting, as some did. The farmers whom the steppe people encountered had just gone through a period of climate change and failing crops. Of course they were malnourished. Does anyone think the people of the advanced Chalcolithic civilizations of the Balkans, creating these large towns, elaborate houses, magnificent pottery and copper ornaments were falling over from malnutrition for most of their history?
Then, just look at this logically. If hunter-gathering was so optimal and they were so well fed, and farming was not and they were not, why did farming cultures produce so many more offspring who survived to reproduce than did the hunter-gatherers? It's obviously because there were fewer resources. Now, whether the children died of starvation, or they were deliberately killed to keep numbers low, I don't know, but as LeBrok mentioned, they did not have birth control.
I also don't get this they weren't adventurous stuff. They spread all over Europe, North Africa, the Near East, India, and down into deepest Africa. That seems pretty adventurous to me. Of course, needs must.
Also, by the time the Neolithic farmers arrived, the Mesolithic people of Europe were sedentary, don't forget. We're not talking about Mammoth hunters. Again, there's all this conflation of time periods. Those Iron Gates people were tied to that fishing area. The hg people of the far northeast were tied to the Baltic shore lines. The hunter gatherers of western North Africa were tied to a particular area because that's where all the hazel nut trees were located.
There is no question the population of Mesolithic people of Europe was small. You just need too much territory to support such a lifestyle for it to be any different.
I've posted this before, but it bears repeating:
https://www.youtube.com/watch?v=SQtzwoOYrkE