As Roger said, changing the power will not change the ratio, or spill. Make sure you've got the strobe mounted appropriately in the umbrella - are yours shoot-through, or reflecting? If reflecting, are the backs opaque, or are you getting scatter through the back? If reflecting, I've found that having the strobe too far out reduces my control as some light escapes the umbrella - but that's also a function of what reflector is on the source.
It was my experience that the scatter and spill from umbrellas caused the extra "broadness", and that could be controlled by using dark materials around the shooting area and moving the subject away from reflecting surfaces.
FWIW.
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"do you like it and think this is the best you can do with the subject and location and model and equipment?". - Glenn Usdin
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