Re-reading The Goblet of Fire to keep up with DD2's questions!
Hagrid - Half giant, half human.
Hagrids Dad - "Tiny little man" There is reference to a photo of an 8 foot Hagrid at age 11 towering over his very small father.
Hagrids Mum - Giantess of about 20 feet tall or so.
Assuming his dad was only "tiny" by giant standards, and he was actually 6 foot tall that still leaves a difference of 14 ish feet. And of course, we have to take into account the difference in size of genitalia, and the fact that giants are well known to be violent and aggresive.
So, tell me how it was done!