Consider the process on a system shown in the figure. During the process, the work done by the system.
We know that,
Work done = force ×displacement
Volume = area ×displacement
Therefore,
Work done=pressure 
volume
Let change in the volume of system = ΔV = V2-V1
Pressure =P
Thus, work done by the system W at constant pressure
W=PΔV
From the graph we can see that V2 > V1 i.e. final volume is greater than the initial volume.
So, work done by the system continuously increases.
1