As surreal as it may sound, Sephiroth, Kirby, Olimar and Pikachu all weigh the same in Smash Ultimate, and it actually makes sense gameplay-wise. As you may know, the weight of characters in the game determines how difficult they are to throw off the stage.
The heavier a character is, the harder it will be to throw them, but the slower they will move. The same goes the other way around; the lighter a character is, the easier it is to throw it off the stage, but it will move much faster and be harder to catch.
Thus, the weights in the game cater to the character’s movements to make them balanced, rather than a question of what their bodies look like. Currently, Bowser is the heaviest character, while Pichu is the lightest.