If a soap bubble expands, the pressure inside the bubble:

The excess pressure $\Delta P$ inside a soap bubble is given by:
$$\Delta P=\frac{4T}{R}$$
where $T$ is surface tension and $R$ is radius.Total pressure inside $P_{in}=P_{atm}+\frac{4T}{R}$.As the bubble expands, $R$ increases. Since $R$ is in the denominator, the term $\frac{4T}{R}$ decreases. Thus, the total pressure inside the bubble decreases.