As of April 2008, banelings have been increased in size from previous builds to make it easier to focus fire on them. In addition their splash radius was increased. The supply cost was higher.
One dilemma with the baneling throughout the development process was the display of team color. Since the explosive ability of the baneling yields green corrosive acid the predominant body color was green and completely overshadowing the small areas that were team colored. Identifying multiple zerg player ownership of banelings was problematic. Later an attempt was made to have the glowing bubbles on the baneling completely team color and even adjusting the explosion color to match various team colors used by players. This quickly made multiplayer look like a ridiculous Jackson Pollock painting whenever multi-colored banelings were seen exploding at the same time. After other various adjustments, such as just the front two bubbles being team colored, the decision was made to fade the team color along the body, with the forward bubbles being pure team color and the large rear bubble remaining green.
↑One of the reasons the size of the Banelings were increased from the size in the original Protoss Release Video was to make it plausible to defend against it. Before, if you have a group of Banelings attack with Zerglings, it was near impossible to focus fire on the Banelings. In the current build, their size, splash damage distribution, and hit points have been increased. These changes still make them potent, yet still allow the possibility of defense with superior micromanagement.
The final size of the Banelings still may change, but this is the current design they are testing. Karune. 2008-04-14. Baneling model - too large. Battle.net StarCraft II General Discussion Forums. Accessed 2008-04-14.