But remember, in 1.0 the trees of the Black Shroud really were massive - the opening scene alone for Gridania in 1.0 literally started with the player walking amongst the trees, and the player is tiny by comparison - here's a screenshot of the scene in question which clearly shows just how big the trees in the Black Shroud really were, with Enki dwarfed by it (the tree is so huge, it doesn't even fit into the frame of the scene - all you can see is the bottom of the trunk):


So yeah, the size of that tree alone does explain the massive length and bulk of the Matron's Lethe.