This one will require you to select the Little and Large Mutation and inflate and deflate your head completely.
To select the mutation, stand in front of the Kinect and wait for the Little and Large symbol to appear on the computer screen.
If you are playing with two people, either person can select this mutation but only the primary player (the one signed and recognized as Player 1) will earn the achievement. To select the symbol as Player 1 stand to the left until the sign is completely filled green. If you are Player 2 stand to the right until the sign is completely filled green.
Allow a few seconds for Mutation Station to create the mutation and your head will become inflated.
The way to deflate your head is simply to use both of your hands and hold them in an upright position on the left and right side of your head. The palm of your hands should be facing your head. Be sure to keep your hands about 1-2 feet away from your head and slowly bring them in closer as if you are trying to mash your head (but don't actually touch your head). As your hands get closer to your head, it should start deflating to the point where it is ridiculously small in proportion to the rest of your body. To re-inflate your head, bring your hands away from your head. It should start to re-inflate again. Do this 1-3 times in a row and the achievement will unlock.