You can reduce the electron app size by packaging using electron-builder package.
PicArt is an electronjs app which I developed recently. It is built using reactJS.
Initially when I packaged the app using electron-packager the Window's build size was
around 98 MB.
Then I found this awesome boilerplate electron-react where they configured the
electron-builder to produced optimised build size.
After using those configuration setup, PicArt's build is now around 36 MB.