I’ve told you, Leeds are a bigger club because they have a much bigger fanbase. It’s as simple as that.
Newcastle are a big club, just not as big as the Geordies think, which is why it’s funny to buzz off them when they get relegated.
It’s an arbitrary line because it can never be scientifically defined or proven in any way yes, but like I say a team with a much bigger fanbase than another will be a bigger club. When the fanbases are similar it comes down to trophies, sustained success, current status.
My answer isn’t ‘because people think they are’, I explained why I think that. You brought up ‘because people think they are’ and yes, relevance is part of being a big club.