Specifically the "black people have massive dicks" stereotype, while sounding like a compliment, was actually invented and spread by missionaries, to emphasise their animal-like nature and thus provide a moral basis for slave trade.
Indeed. This comes a little later, but as an example of a frankly horrendous text written, this kind of thing illustrates some of the thinking that went along with those kinds of attitudes - http://losinger.altervista.org/documents/HowardMedicine.pdf
(some direct quotes)
When education will reduce the large size of the Negroís penis, as well as bring about the sensitiveness of the terminal fibers which exist in the Caucasian, then will it also be able to prevent the Africanís birthright to sexual madness and excess.
The truth is that the negro of today, untrammeled and free of control, is rapidly showing atavistic tendencies. He is returning to a state of savagery, and in his frequent attacks of sexual madness, his religious emotionalism, superstition, and indolence, is himself again - a savage. This animalism, this innate character of the African, will demonstrate itself more and more as his is allowed the liberty of his sway of ancestral sexual impulses...'
That was written by a fucking doctor in 1903. Next time someone says 'I don't know why anyone would complain about being told they've got a big dick', people should have some fucking awareness where this drivel and these awful associations come from.