The new Mail compose window began rolling out to UMICH users in April of this year and is now the default for M+Google Mail. Some users who had reverted back to the old compose during the transition period have begun seeing their interface change to the new compose experience. This is an update made by Google and cannot be changed or turned off by U-M's Google Apps administrators.
However, you can control the size and placement of the default window. If you prefer to have a large compose window centered on your screen, hold SHIFT as you click on the COMPOSE button. If you have Keyboard Shortcuts enabled, press SHIFT+C to open a larger window. You can also pop out the compose window by clicking on the arrow in the upper right of the window.
You can make the full-screen compose window your default by selecting Default to full-screen in the more options menu in the bottom right of your compose window. Google added this new option in July. (Note that the default is only for composing new messages -- it doesn't carry over for replies or forwards.) Read more about how to enable the full screen by default.
Here is more info about the compose window: