The Sequoia is many times as large as a blue whale, more than ten times as large. The coastal redwood is also far larger, and there are no doubt other trees that are far larger than the blue whale.
But what we are looking for is the largest living thing, and trees are actually about 99 percent dead. This might seem odd to you at first, but remember that your hair and the outer layer of your skin are dead, they are composed of dead cells. A fair amount of your skeleton is not inside any living cell. So a lot of you is not actually living cells. For trees, the nonliving portion is much greater. Most of the trunk and branches is composed of dead cells. For trees in general only one percent is alive, for redwoods, I think it is likely the portion that is alive is even smaller.
So if we look at the weight of the living cells of a blue whale, versus those of the Sequoia we would find that the blue whale is probably somewhere in the very rough neighborhood of ten times the size of the Sequoia or coastal redwood.
It has been argued that a giant fungus is even bigger. It extends over four square miles and may weight even more than the aspens.
But this is different than the blue whale and the Sequoia. Occasionally human twins come out joined together. Suppose two twins joined at the hip became very obese. Together they might be the world's heaviest person. But most of us would consider them two people, not one. This is more or less what we have with the grove of aspen or the giant fungus.