OP said less than 40MB. You can build Slint binaries targeting embedded devices. So a few hundred KB for a small application. Still arguably too much space but

OP said less than 40MB. You can build Slint binaries targeting embedded devices. So a few hundred KB for a small application. Still arguably too much space but gui frameworks like slint use a domain specific language that generates more verbose code than what a human would typically write. Probably in the interests of being generic.