A hydrogen-inflated polythene balloon is released from the surface of the earth. As the balloon rises to an altitude up in the atmosphere, it will

As the polythene balloon ascends the ambient (atmospheric) pressure falls; for a fixed amount of gas the ideal gas law (PV = nRT) implies that a decrease in external pressure leads to an increase in gas volume (so the balloon expands) . High‑altitude balloon design explicitly assumes this expansion — balloons are treated as inflating to much larger volumes at low pressures and are often made to accommodate that change [1]. Practical experience and technical reports note that free‑expanding polyethylene/weather balloons increase in size with altitude and can eventually burst if the material limit is reached [2]. Thus the balloon will increase in size as it rises, until limited by the film strength or valve design .

Sources

1. [1] https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/designing_a_high_altitude.htm
2. [2] https://apps.dtic.mil/sti/tr/pdf/ADA064126.pdf