A clean instance of Opera, which has all bells and whistles enabled, eats roughly 300MB of memory. The desktop version of Safari appears to be in the same ballbark.

So, 512MB of total system memory might be a bit tight while browsing so the 1GB memory requirement sounds quite plausible.

For Iris, it might actually make sense to stick with the older Webkit. It is not like HTML emails would require the latest generation of browser engines to display properly. (Thankfully.)