Other people have covered the bandwidth angle, but I also think the size itself is an issue because of memory. And the more Electron can load into memory out-of-the-box or via default coding conventions, the more opportunities for poor memory management from developers. Since Electron is being used for utility apps, its bloat multiplies on a system with a lot of apps on it.
Also, Electron's size means more power draw. When I want enough battery life to do some work and still watch a long video afterwards, I habitually close Slack because it makes a worthwhile energy difference. The app's size reduces my engagement with the app after install. I'm aware my behavior is unusual, but developers do need to keep this in mind, especially as Windows and OSX make app energy use more transparent.