Tk is just a shared library accessible by any compiled language; there are many C/C++ cross-platform applications using it; it was actually also the first GUI toolkit used by Python; it's true that it includes Tcl which is a scripting language, but that doesn't matter much. It's nearly as lightweight as FLTK and Fox toolkit.

Applications AOT compiled with the Flutter SKD are surprisingly lean (i.e. compared to Node/Electron) and not much larger than Qt applications.