GT is the only one application that change the window size depending of the tab. That doesn't feet with GNOME general usability...
All I'm going to say refers to the window-mode, and no to the full-screen-mode (which problematic is opposite). But only devs use there terminal in full-screen, and I talk for other people.

The font is actually associated with each tab, and not with the window. So, if you create a new tab with a different profile (for another use), it's created 80-chars-width, and if the font is different, the window is resized.

The solution is to change the profile-use, having a "WindowProfile" (simply called Profile) and a "TabColorSet" ("colorset", maybe...) :
*When you create a new window, you could choose your WindowProfile, which describe the default font, and a default TabColorSet
*In each tab, you can change the TabColorSet
*In each tab, you can change the number-of-caracters-width, which egaly change the font to fit to the window

With this method, you have the choice to use TabColorSet and/or WindowProfile, depending of if you're a big terminal-user. The size of the window depends on the WindowProfile font, and the "zoom" applied (it applies to each tab). A tab-font depends of the zoom the size of the window it is in (see previously), and the number of characters-width (by default, 80).

Other information:
I join a file discribing the menu-bar of the proposed terminal.

See egaly bug 412056 : http://bugzilla.gnome.org/show_bug.cgi?id=412056