I’ve always lived in the belief that it is signature radius with some small variables…
There have been multiple posts throughout Eves history about the topic and what i remember 100% surely seeing is some official statement on how the overview doesn’t apply MWD or other variables with the reasoning being that it gives away too much intel for pvp like shield extenders and shield rigs.
But you’re right it doesn’t still explain the difference in signature radius vs. size if it’s supposed to be without effects applied to the stat… 
I want answers!
Here’s one post where someone claims that MWD affects the overview but it really doesn’t though… Then everyone blames size column being broken and we have no answer.
https://forums.eveonline.com/t/trying-to-figure-out-how-big-the-ships-are/408790/15
EveUni claims that it’s just the size of the ship in meters. But thats wrong too according to the threat above and show info on ships vs overview.
https://wiki.eveuniversity.org/Overview_settings