Most modern browsers cache a lot of data in memory as well to make things like navigating back snappy and avoiding a full page re-load. It's also necessary to cache most of the state of the previous page to allow retaining form fields if you accidentally navigated away (as some forms are only in the DOM, created by JS, and not part of the original HTML).

I believe to have read somewhere that at least Chrome listens to low-memory situations broadcast by the OS and will evict such caches. So while it uses a lot of memory as long as memory is available, it will also release much of it if necessary.