I mean, that's porn which is kinda naturally focused on extremes instead of averages. Not to mention strong promotion of exaggerated/false body image and such -- going by what you see in porn you'd think that 95% of women have at least DD tits and implants.
But as far as actual research goes,
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. Which, if it was true, should be easy enough to demonstrate, no...?
Not to throw a shade on your nurse friend here. But aside from the small sample as you mention, there's also potential confirmation bias; if someone already believes "black guys are the biggest" they'll subconsciously filter out cases which don't support their views and remember those which do.