Continuity mistake: The clown balloon was swelling with the Smilex gas, but when Batman steals it and flies it and the other balloons away, it's back to normal shape.
Suggested correction: The balloons were not filled with Smilex gas. The Smilex was in canisters that are visible underneath the balloons. The balloons themselves could have expended and contracted due to varying air pressure.
Yes the canisters were on the underside. But if you look closely, when activated, the balloon began to swell, and holes appeared, releasing the gas.