I was thinking this yesterday: why are plants so much bigger than animals? Is it just that it's easier to be big when you're standing still and have strong cell

I was thinking this yesterday: why are plants so much bigger than animals? Is it just that it's easier to be big when you're standing still and have strong cell walls? Is it because photosynthesis makes it easier to scale nutrition up? A blue whale is an exceptionally large animal and it's a rare megafauna, but a tree that is as long as a whale is a totally normal and boring thing. Why are they so much better at being big?