How do you make the link between app size and power draw (as opposed to some other aspect of Electron causing draw)? And, what are you comparing Electron's power draw to?
A larger application presumably uses more RAM to itself (especially if its instances of node/chrome/libraries are distinct and so do not share pages by reference or in a copy-on-write manner with other similar applications) so there is less room for cache and buffers so the machine as a while is likely to be hitting disk more often and so consuming more power that way.
Loading the app will obviously involve more IO too, though hopefully that is not a regular occurrence for a full desktop app (it may be for a smaller utility program though) so the running costs should dwarf any start-up costs over time.
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.