My brother in Christ, the majority of full nodes doesn't decide anything in Bitcoin. Bitcoin was explicitly designed to resist Sybil attacks, yet one of the most common misconceptions in the space is that a majority of full nodes can enforce consensus. There is no voting in Bitcoin, except in the sense that hash power represents a vote. (Per the whitepaper, one CPU, one vote. Now, one ASIC, one vote.)

Any number of full nodes can activate a fork if there is a failure to achieve consensus. The number of nodes involved has nothing to do with the mining hash power or economic value of that chain.