Complete reboot of KerBalloons here
Currently the balloons are inflated by button/action group.
As the air pressure around the balloon decreases the balloon expands, and as a result the lift decreases (as shown in the graphs below).
When the balloons expand to their limit, they burst. The one pictured currently bursts at pressures below 4kPa.
Planned Release Content
Release 1 - Concept release
1 size balloon
Release 2 - More parts
More, different sized balloons
Small electric propellers to control bigger balloons
Some smaller, lighter science parts
A small service bay
Release 3 - Contracts
Various contracts for the balloons
Release 4 - Tethering
Balloons can be tethered to the ground
More info on the Dev Thread