I remember hitting a similar limit in webOS. There's a WebKit image cache setting set either at build time or startup time about how large of an image will be allowed to persist in the internal decoded image cache. When an image exceeds that, the decoder produces a scaled down version that's grown when displayed. We could adjust it on our platform with a setting in /etc/palm/browser.conf, but it wasn't adjustable without making the core file system writable and restarting the system manager.