They offer web optimized versions. The full variable font in woff2 comes in at 523kb, and then there are subsets available from there (reduced character set, reduced customizability), and you can even build your own font with only what is needed.
Generally with web design, you want to stick to a small number of font weights anyway so if the file size isn't smaller, what's the core motivation?
The key thing also is the number of web requests. This is not a trivial detail. One variable font can be less web requests.
Is the number of web requests a factor if you're using HTTP/2 though?