Why do I have less internal storage space after moving Google+ to my SD card?
Version 1.0.7 is significantly larger than previous versions. Even after moving to the SD card I'm showing 14.5MB taking up system storage. That'll account for your 2MB reduction in space.

Google generally hasn't allowed for their apps to be moved to the SD card, presumably so they can be sure widgets will work or other performance considerations. However, since this version weighs in at over 24MB I guess their hand was forced.