Capacitor energy stored in relation to pd

Why is the energy stored in a capacitor proportional to V^2 and not just V. As there are two equations for it E=0.5QV and E=0.5CV^2. From what i see, E is proportional to V and V^2 and in the mark scheme its V^2.
Can someone clear this up for me, cheers.

Q isn't a constant. Q is proportional to V, by CV = Q. So there's a hidden factor of V in Q in the equation $E = \frac{QV}{2}$.