React comes in around 26k, Flux is another 1k or so, and say you throw in underscore that's still something like 30k combined. Add another 5k if you need an es5 shim / sham (say you're supporting earlier IEs). Another 4k for superagent. So it looks like somewhere around 40k minified and gzipped to run a reasonable React app.
The nice thing is that you don't even need jquery once you're in react land, unless there's an addon you absolutely must have and would be a pain to reactify. We haven't had to use jQuery in our last few projects.
That's 3X though and not 8X. 8X sounds like a biased exaggeration. I don't understand why people have to polarize so much over js frameworks...