I always wondered why they did that to Servo. From the MST3K wiki:
"This physical form was kept throughout the remainder of the series, except for a brief flirtation (during episodes #205: Rocket Attack U.S.A. and #206: Ring of Terror) with a slim cylindrical gumball-machine head to try to reduce the screen area Servo's head obscured. It was introduced as a "haircut" that Joel gave Servo, but was quickly abandoned."