seeing how well Gantu took to the growth experiment, Jumba was eager to chug down double the amount to make him REAL BIG! but if he knew what would happen to drink so much of it, he would've kept it to one botlle.
now he's one massive mountain of blubber for everyone to see, which wasn't exactly the attention he planned for. and he's too big to go back in the lab to fix it, so he's pretty much stuck like this.