The best data on this probably come from Lui Kotale; see e.g. Hohmann & Fruth 2002 Dynamics in social organization of bonobos in which they relate variation in party size to food availability. Frances White has similar data for Lomako (White 1996 Comparative socio-ecology of Pan paniscus). 

In general, 'cohesiveness' is determined by food availability across primates (see Sterck et al 1997), especially great apes who are large-bodied and therefore 1) experience strong feeding competition because they need to eat a lot and 2) are not very vulnerable to predation, and hence don't need be in a group for protection. Both factors mean that they will respond to food scarcity by splitting up and to abundance by coming together (resulting in their typical fission-fusion societies). The extreme cases are orangutans, who are forced to forage solitarily most of the time because the forests are not very productive, and bonobos who experience relative abundance much of the time and can therefore afford to be together. Chimpanzees are somewhere in between, depending on the population as there is a lot of variation within each species, with higher food abundance predictably leading to higher gregariousness. 

Rebecca Stumpf has a good summary chapter on chimpanzees and bonobos in Primates in perspective: Chimpanzees and bonobos- diversity within and between species