As one of my instructors once said; "You can't trade punches with someone twice your size". Of course there are those you can't "fight" successfully, but Self-Defence is about FAR, FAR more than fighting. Avoidance first, Escape second, or, failing that, quick strikes to vulnerable areas is the key.
There are people without a day of training who could take my head off - and I began martial arts training in 1972 at the age of 5. However; I don't rely upon my ability to "slug it out" with neanderthals. My mind is my greatest weapon - followed by a sidekick to the shin or knees and a palm-heel strike...
Of all the RBSD folks out there, Geoff Thompson seems, IMO, to have the best information on this scenario.