The book "The Creation of Inequality" (Marcus & Fannery, 2012) says the Magdalenian groups were much larger than the Gravettian groups but doesn't provide a clear number (or I did not find them). Any idea how big they were?
Actually, the truth is that - in absolute terms - we don't know the size of the groups. All that we are able to say is that the material culture of a particular group - as revealed by archaeology - suggests that the size of a given group may be larger or smaller than some other group.
Occasionally we have group burials from a site like the Maszycka Cave, which do allow us to make a (very) rough estimate of the population at a particular site, at a particular time, but we really can't say how representative those estimates are of the wider culture.
It does appear that Magdalenian groups had more permanent settlements (like those at Petersfels and Schussenquelle) than earlier Palaeolithic groups. This suggests that relations between the various Magdalenian groups might have been closer than those between more-mobile groups of earlier periods (like those of the Gravettian culture).
Archaeologists often infer that these more permanent settlements would have supported larger populations. If you are interested, the topic of Magdalenian settlement patterns is discussed in some length in The Magdalenian in Western Central Europe: Settlement pattern and regionality by Gerd-Christian Weniger.
However, recent work by Aurélien Simonet (Gravettians at Brassempouy (Landes, France), 30,000 BP: a semi-sedentary territorial organization?), and others, has suggested that the people of the Gravettian culture might have been more settled than was previously supposed. In turn, this may lead to assumptions about relative population sizes being revised.