Answer
33 percent
Work Step by Step
Use the ideal gas model. The amount of gas, n, and the temperature are constant. The value of PV remains constant. The pressure inside the bag equalizes to the pressure of the airplane cabin.
$$ P_1V_1= P_2V_2$$
$$V_2=V_1\frac{P_1}{P_2}$$
$$V_2=V_1\frac{1.0atm}{0.75atm}=1.33V_1$$
The bag has expanded by 33 percent.