On one hand, it would probably mean that science/technology advance quicker, since there would be more people to get geniuses out of. On the other, the overpopulation would lead to worse overall sanitation and maybe to faster colonization, as there would be more people willing to get out of Europe. Also, the fact that the relative population of Europe would be larger when compared to that of the Muslim world (not sure if this is the case, but I think Europe was affected much more than Middle East / North Africa) might play a role in the Reconquista and similar processes.