Bitcoin is not the heaviest valid chain, and there is no “definition” of Bitcoin either.
It is a vague term that exists because of a general consensus all sorts of participants, which have weight in different ways (economic, computational, leadership, publicity, etc.)
And there is no strict weight to define 50%+1 but many different powers that form a result.
A node by itself does not have any weight in this, and only its operator could hold weight by being economically active (buyer/seller/platform), mining, via public participation in discussions, etc.
