While it’s true that Svelte and Lit can grow in size dependent on project there’s no world in which even large projects get close to the base level of the React runtime.
I have no idea about that but if the figures you’re providing are correct it’s pretty obvious the answer is that they did it wrong. There is nothing in the web component API that would require 100+ requests nor several MB of JS, especially when you’re in control of every step in the process!