How fat or skinny of wrapper is gtkmm?

It's pretty skinny. Looking at some of the tutorial examples, after strippig the (dynamic) executables, we get the following sizes (on a RH 6.0 system, using egcs 2.91.66, not using any Gtk themes):

Gtk vs gtkmm resource consumption comparison

ExampleGtk+ File SizeGtk+ Memorygtkmm File Sizegtkmm Memory
buttons62561844309962820
clist77201900320122840
filesel54962076278203052
progressbar98441916340562932
rulers66321812243002740

where GTK+File is the file size and GTK+Mem the resident memory size of the GTK+ executables; and gtkmmFile and gtkmmMem the respective sizes of the gtkmm executables.

C++ executables are necessarily larger than C executables since they require all sorts of info in order to properly link (dynamic) class structures at run time. All Unices today are clever enough to actually load in memory only what is needed. Therefore, a large executable and large dynamic libraries do not necessarily mean large memory consumption.