As
Quizro pointed out, a lot of the action scenes were of the
Japanese Rangers, a group that included only one
female (the
Pink Ranger). However, since the American group required more
diversity to be
palatable, they decided to make the
Yellow Ranger be a
woman as well (and asian at that for a "politically correct" racial mix).
Now the problem was the action
sequences. They needed a
flat-chested enough
woman, so that the difference in the
Yellow Ranger would not be noticable to the audience, but especially
children (as if that was a problem
anyways). This was actually a problem during
casting, and thus that is why you wont see the Yellow Ranger's
character wearing anything that would accentuate her
chest. The newer ones actually have a female stunt person in the
yellow suit, since some are now filmed exclusively for the US.
So if you ever catch an older one, keep a good eye on the Yellow Ranger. It's a guy.