I also don't understand the styling complaint. React devs used "just CSS" for years before some of them started pushing for CSS-in-JS. In fact, SCSS/CSS is the default for the still nearly ubiquitous create react app system.
End-user bundle argument is a red herring. What is true for "hello world" isn't true for large applications. The overhead of React is a one-time payment while each new Svelte component drags along all the code needed to support it (copying similar code over and over).
React's F + N * C (where F is framework size, N is component count, and C is average component size) is smaller for large values of N than Svelte's N * (F + C) approach.