Something to consider is how your leg muscles are compatible to the fins, especially with large, stiff fins.
If possible swim with fins under consideration while someone observes your kick.
Your legs should be as extended as possible. I use a visualization exercise (
Visualization ) that has proven effective.
Have someone swim behind you and look at your fin stroke. If your legs have the muscle to properly drive the fins, the fins will move in an up and down motion giving the best forward movement with each stroke. If the fins are too stiff ( a quite common occurrence), the fins will "slip" and move side to side during the kick stroke. This "slippage" is wasted energy and promotes leg cramping.
Next have someone swim beside you. Your legs should be extended (your legs are a lever and the longer the lever, the more energy is used for propulsion). If the fins are too stiff, the leg muscles are not strong enough to drive the extended legs ... from the side, it will look like you are riding a bicycle (cleverly called a "bicycle kick"). Again, this wastes energy and promotes leg cramping.
Wasted energy translates to decreased bottom time.
The above exercise takes a bit of time, but in the long run, matching legs to fins provides longer bottom time with less fatigue.