Safari, like any modern web browser, simply uses a lot of memory. Remember that mobile Safari is (almost) always running in the background and on 128 MB device

Safari, like any modern web browser, simply uses a lot of memory. Remember that mobile Safari is (almost) always running in the background and on 128 MB devices it is always consuming most of the available RAM despite how aggressively it tries to flush things from RAM. Caching to disk alone isn't enough. All web browsers cache to disk, but most non-trivial web sites will still hit the network as they are rendered by the browser, even if the content is cached. And if the network is slow you'll be waiting a while for that "cached" page to appear. Safari would need some kind of off-line browsing mode, like what I think Firefox has.