The vowel chart of female is boarder than that of male. Are the two systems overlapping regardless of scale, i.e. by dividing by a number? If not, what are the differences and what does that mean?
Because females typically have a shorter vocal tract compared to males, one might expect that you can normalize formant frequencies to factor out differences owing to physical differences. This paper suggests, contrarily, that gender-based formant differences are grammaticalized and language specific. If formant differences in vowels were the result of otherwise identical gestures with vocal tracts of different lengths, all formants would be affected. In fact, F1 does not differ between male and female Parisian French speakers, but F2 and F3 differ (those formants are significantly higher for females, compared to males). Moreover, that expected difference with F1 is found with (Northeastern) American English male vs. female speakers. It is also reported (citing Johnson 2005 "Speaker normalization in speech perception") that gender differences are small in Danish and large in Russian. This study of Marathi suggests that the different ratios can be vowel-specific within a language.