i see you also bend your knee a lot, i would fix this first, the ankle should follow (not 100%, but more than a bit)
Vali is right, knees are wide and this comes from you: i bet you felt you fins touching each other the first time you wore them!!
try this: while finning THINK of your legs as if they have no knee and try not to bend it, your legs have to be straight and rigid :crutch
this is not what you actually want to achieve, but it helps reducing the bending of the knee
after that you can go for the ankle (see first posts):ko
Thanks sgnips!