As the title says, but surely that's not right just for the launcher?

The itch app executeable alone is 120 MB. 200MB is nothing. The itch app is a Chromium browser if I recall correctly. It is downsized. Real Chrome is maybe 800MB.

The Steam "launcher", that is also a modified browser, is around 600MB.

Which launcher are you comparing it to, that you think, it is big? Oh, wait, you did not explicitly say it is too big. Maybe you think, it is too small ;-)