Behind the full nodes are people and they can choose to run whatever code they want.
So? What's your point?
My point being is that the longest valid chain will be decided by the people who are behind the full nodes and miners. Therefore if they choose to support forking Bitcoin in order to increase the blocksize then that will become the longest valid chain, even if you still personally do not consider it valid it does not change the fact that according to the terminology that we are using it is.
Technically they can, practically it depends on whether the majority of nodes actually represent the economic majority or not, since nodes can easily be spoofed, this is why proof of work is so useful since it is a better measure of consensus because it can not easily be faked.
It doesn't matter that nodes can be spoofed. What matters is the nodes being run by the ones holding sizeable investements into the system. If the miners try to go against their will they will be bankrupted.
It most certainly does matter that full nodes can be spoofed, this is why proof of work was necessary in the first place. It is simple really, participants in the network have the free choice to support any code that they want, therefore consensus is not totalitarian, it is more democratic in nature. People have the free choice to support whatever code they want to, which will in the long run determine the future of Bitcoin.