Explanation:
Mylar balloons are different from usual latex balloons. Mylar balloons are made of Nylon with metallic coating. The latex balloons are porous so they do not stay inflated for a very long time while Mylar balloons are not porous so they can stay inflated for a longer time.
When Jessica took the balloon outside the gas inside the balloon started expanding. But the expansion was due to the expansion of the outer layer of the balloon itself. As metal expands due to heat, the balloon got bigger.