For some of the cells, like neurons, that to a first approximation don't divide in adulthood, this seems true. But for things like blood cells that are constant

For some of the cells, like neurons, that to a first approximation don't divide in adulthood, this seems true. But for things like blood cells that are constantly getting turned over, it seems like there would be a lot more opportunities for bad mutations in a larger creature (although as your link suggests, there seems to be some fairly strong defense against this).