The Grinch has a very small heart, which is not much of a surprise given just how much he hates Christmas and everything it stands for. All alone on his mountaintop redoubt and with this shriveled heart beating beneath his ugly green skin, the Grinch presents a truly pathetic figure. With a heart so small—two sizes too small, to be precise—it's no wonder that the Grinch can't experience any love or kindness towards a single soul. There's just no room inside that withered heart of his for anyone else.
Although we never get to discover exactly why the Grinch hates Christmas so much, his abnormally small ticker would seem to provide the most likely explanation. Hating Christmas is one thing, but it takes a special kind of heartlessness to try to steal it altogether as the Grinch does. There may not be enough room in his heart for love, but there's certainly enough for hate and lots of it.
Thankfully, the Grinch comes to see the error of his ways. The problem was that he never really understood the true meaning of Christmas. But when he finally gets what it's all about one Christmas morning, he starts loving the festive season as if his very life depended on it. Not only that, but his shriveled heart starts to grow, three sizes, to be precise. For good measure, he now has the strength of ten Grinches, plus two.