I loaded Yahoo Mail and Gmail in Firefox 2.0 and Windows Live Mail in IE7, as it had problems loading in Firefox.
Gmail: 1275x612 page -> 847x457 mail content (49.6%)
Windows Live Mail: 1275x622 page -> 847x386 mail content (41.2%)
Yahoo Mail: 1275x612 page -> 914x321 mail content (37.6%)
In Yahoo Mail you can open a message in a new tab, while in Windows Live Mail you can hide the list of messages. But, by default, both sites devote much less to email's content than Gmail. As a comparison, Mozilla Thunderbird uses 51.3% from the window size to display message. So, although Yahoo Mail Beta and Windows Live Mail try to look similar to a desktop application, they neglect an important aspect: how easy is to read mail?
Note: These numbers were obtained using the default settings and 1280x800 resolution.