What is the effect of packages installed by snap on memory usage?

Snaps are indeed sandboxed applications that run in a confined environment, and contain, to an extent, their own libraries. These libraries that come with the snap cannot be used by other snaps. Therefore, memory requirement of multiple apps will be higher than in a system where these libraries can be shared by different applications.

Still, also within snap, some sharing is implemented through first of all the core snap, but also through optional use of extensions that avoid that the developer each time has to implement integration with for example Gnome and GTK, or KDE in the snap, and leads to reuse of these components between different snaps.