I think that the contents of the emails in a thread is a minor part of 150MiB taken by the full version. I suppose my entire inbox (the not-archived part) is much smaller that that.
Indeed, it's not the data but the code to support it. There's a bazillion features in there designed to avoid the user having to do additional page loads. For example in my Gmail memory profile there's 20+ MB of code to support the real-time chat feature in the sidebar. You can argue about whether there should be a middle ground implementation that has the email preloading but not the real-time chat. Developer and project manager time is unfortunately finite.