The Grinch doesn’t become a fucking socialist. He was a grinch because he was anti-capitalist communist and didn’t want people buying each other gifts. He was against the commodification of love and family by way of cheap trinkets. He was against the idea that you could buy love, and thus plotted to steal all of the presents from Whoville and force the Whos into a situation that would force them to love their family for their company and their love, and not for whatever that person bought them, just for one day. When the Grinch saw how the capitalist culture of Christmas brought actual joy to Cindy Lou Who, he realized that weighing his own philosophy on a whole society by theft was misguided, and he had to weigh his forceful revolution against the genuine happiness of the society. This lead him to the conclusion that his morals are not universal, and that the most important thing for people is whatever brings joy into their lives. As far as Whoville and the Whos are concerned, the Grinch embraced capitalism.
It wasn’t the Grinch’s heart that grew three times its size, it was his brain.