For elastic action, the hoop stress is P/(Rt), where P is internal pressure, R is radius, t is wall thickness. Calculate hoop strain from strain = stress/E, and calculate change in radius = strain x Radius.
This is the uniform radial enlargement due to pressure. I wouldn't call it "ballooning" as when that happens in a balloon, it is highly non-linear. I doubt Formulas For Stress and Strain covers the inelastic case, as that would be dependent upon the stress-strain curve.