Tiny C++ cross-platform GUI toolkit

the smallest one I've heard of is fltk

The Tk from Tcl/Tk is well known of its small memory footprint. Details in Memory Footprint Comparisons on Tcl/Tk Wiki. Both, Tcl and Tk, are extremely simple to use with C++/Tcl and C++/Tk libraries. They were inspired by Boost.Python

With the Ecere SDK, you can build a statically linked executable under 1 MB. A dynamic runtime library can also be built under 2 MB. Ecere runs in a variety of display drivers, including X11, GDI, OpenGL, Direct3D. It was originally designed for in-game GUIs, but it's now a rich general purpose cross-platform GUI toolkit.

The SDK comes with a compiler for the eC language, an OO language which compiles down to native code, that can be linked with C++ at the C level (i.e. extern "C").

eC shares many basic OO concepts and syntax of C++, but offers simpler, more elegant code, especially for GUI development: it has properties, instance virtual methods for events, you use '.' instead of '->' for accessing heap objects members. It still fully supports the entire C syntax, grammar and features. eC rids you of the pain of header files and prototypes as well, with a more modern import mechanism.