Why are weather balloons partially inflated before release?

Question

Why are weather balloons partially inflated before release?

Answer 1

Yes, the balloon is initially filled just enough to lift the radiosonde upward at a sufficient rate.  The balloon expands as it rises, but the relatively small amount of gas to begin allows the balloon to survive up to over 70,000 feet before bursting.

Answer 2

As they rise to where the pressure is lower, the gas (helium, or hydrogen) in the balloon expands to fill the available volume. If it was fully inflated before release, the balloon would burst, at altitude.