I think what squarefoot is looking for is lightweight (tiny?) C apps that have limited abstraction layers. For example, Gtk+, which is pure C, is no longer considered lightweight. It was with Gtk+ 1.2 because GTK was just a small wrapper around X11 in the 1.x days. That is what AzPainter is. AzPainter basically depends on xlib, xft/freetype, and some image libraries (libpng, libjpeg).

Modern C apps with a GUI typically build on GTK but that toolkit has become massive. Modern GTK3 apps are built on top of dbus, pango, atk, cairo. This is not necessarily a bad thing though and libraries like ATK provide accessibility which is important. At the time GTK+ was created the competing toolkit at the time, under Unix, was Motif and it had the nickname "bloatif". Now we've reached a point where GTK3 is much much larger and "bloated" than a 2019 Motif application.

If we include C++ the Qt and WxWidgets toolkits are also massive. FLTK is still a lightweight option though.