Look at these two sprites. Can you see the differences? For the female (right), yes, it's white face is bigger, and the male's white face is smaller. However, according to this it is portrayed that the male Starly have bigger foreheads. Which is understandable, but if you zoom in close enough you can find that isn't really true.
So, your theory is correct. Congratulations for spotting that!
Hope I helped. :)
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