He doesn't, its just a poorly phrased sentence, I thought he meant what you thought the first time I read it.
What he wants is a way to have more cache so less is downloaded in total. However I think that's not going to be much use, because 5Gb isn't much and he'd have to cache such a lot to make a difference that Safari would slow down to a crawl, as it spent a huge amount of time searching through the cache for every single page element. I suppose if its slowed down enough, it might limit his downloads by virtue of that low speed, but that's probably not what he is seeking to accomplish
One way to verify this would be to use Firefox, whose cache can be increased, up it to say 5Gb and see what happens.