If you discard all that claptrap about the gods of Middle Earth creating elves, dwarves, humans, and hobbits, then you must acknowledge it is a simple matter of evolution—that all these Middle Earth species were descended from a common, if distant, ancestry.
In fact, the same thing happened here on the ever-so-mundane planet earth. There have been 14 branches that we know of from the common genus of humanity. Many died out before the arrival of Homo sapiens, but for 190,000 years, we walked the planet alongside other sub-species of our genus.
The genetic cousins whose time overlapped during this 190,000-year period are:
Homo neanderthalensis – In my scientific opinion, this is a younger Gérard Depardieu. In fact there was some crossbreeding back in the day, so that nearly all non-African Homo sapiens get 1-4% of their DNA from Neanderthal. These folks averaged 5’7”, 120-155 pounds and had a larger cranial capacity than Homo sapiens.
Homo rhodesiensis – There is very little fossil record, so no average height or weight, but in what becomes a common theme, their cranial capacity may also have been larger than that of the ultimately triumphant Homo sapiens. Woot-woot for the underdog.
The Red Deer Cave People – These folks were “jut-jawed and heavy browed.” I couldn’t find a reasonable picture to use, but there is an artist rendering in a 2012 issue of National Geographic. Again, very little fossil record on which to base average height and weight.