I won't recount the history of investigations in Lovelock Cave here (you can read a basic outline on Wikipedia). If you Google "Lovelock Cave" you'll get a mixture of results, some focusing on the actual archaeology of the cave and many talking about the Si-Te-Cah legend and the "red-haired giants." Apparently the "Paiute legend" of cannibalistic, red-haired giants originated with a story by a Paiute woman named Sarah Winnemucca Hopkins in her 1883 book Life Among the Piutes: Their Wrongs and Claims (see this 2013 post by Brian Dunning). The part relevant to Lovelock is the last paragraph of Chapter IV. If you read it you'll notice there's no mention of giants: the often-repeated statement that Paiute legends include giants in Lovelock cave seems to be a later addition. I guess it makes it easier to find giants if you just make them up. I haven't spent a lot of time checking into the various legends that are cited as evidence for the worldwide occurrence of giants, but I won't be surprised if a lot of them evaporate when you start to look closely. So far, the giantologists are 0-2 in my book (Lovelock Cave and Steve Quayle's Celtic giants).
"Inside this cabinet here are three skulls from the Lovelock Caves. When you first see these skulls, they pretty much seem to be normal looking skulls. However, it's when we really start to compare the jawbones with this modern dental impression of a normal adult male that we see that these jawbones are unusually large. And these are really the skulls of giant people. Who were perhaps seven, even eight feet tall. One of the odd things with these skulls is that they're not actually put on display here at the museum and they're kept hidden in this cabinet. Now we don't know if that's really just out of respect for Native Americans or whether there's really something unusual about these giant skulls that they don't want them displayed."
The silliness of the comparison between the "modern dental impression" (which includes only the teeth and a small portion of the gum line) and the Lovelock mandible should be evident to anyone who is breathing. It has been pointed out before. The total size of the cast is smaller because it doesn't include all the bone of the mandible. In what we are shown, the comparable parts of the cast the and the mandible (the teeth and the tooth row) do not really appear to be that different in size. Ancient Aliens only shows us the "normal" plaster cast sitting in front of the Lovelock mandible, however, and doesn't actually give us a view that allows a direct comparison.
Both the photos are arranged in the same way, with the plaster cast "inside" the Lovelock mandible, creating the illusion that the mandible is much larger than the plaster cast. Superimposing an outline of the tooth row of the plaster casts onto the mandible shows that, while the teeth and tooth row of the mandible are a little larger, it is not "giant" in comparison to the casts (Terje Dahl points out the same thing on his site, but concludes that that must mean the "real" giant skeletons have been replaced with normal-sized ones).
The illusion of a dramatic size difference is created by the parabolic shape of the human mandible: parabolic objects of similar size can be nested inside one another. Nineteenth and early twentieth century newspaper accounts of "giants" often describe the mandible of the skeleton as being so massive that "it will slip over the jaw of a large man." The uselessness of this comparison was noted by Gerard Fowke in his Archaeological History of Ohio (1902:142-143):
"It is a very common newspaper statement that a Mound Builder has been dug up somewhere 'whose jawbone will slip over that of a large man.' Sometimes the man elevates the marvelous into the miraculous by having a growth of 'remarkably heavy whiskers.'
It is not necessary to procure a Mound Builder in order to perform this feat; the phenomenon is equally apparent with any other full grown human jaw. It may be observed, also, in curved or open-angle objects generally, having approximately the same form and thickness; as spoons, saucers, miter-joints, gutter-spouts, or slices of melon rinds. The significance is a great in one case as in the others. The experimenter has failed to perceive a considerable interval between the end, or angle, of the jaw which he held in his hand and the one with which it was being compared. He should invert the former and apply it to the lower part of the latter, when he would find much less difference than he expected."
Gerard Fowke worked for The Smithsonian, so I'm sure some of you out there will take his basic understanding and explanation of geometry to be part of a vast conspiracy to suppress information about giants. If you're skeptical, I suggest you get some slices of melon rind and try it yourself. Paper cups will also work if you don't have melon rinds or human mandibles sitting around.
The mandibles and skulls of Lovelock Cave are not those of giants, and the "legend" of giants attributed to the Paiute appears do not actually contain any mention of giants. The Humboldt skull does not have double rows of teeth (and neither do any of the Lovelock skulls, if you noticed).
Why does this mythology about Lovelock have such staying power? This is one of the relatively few cases where the skeletal remains of supposed giants have been available to look at. Even when it is perfectly obvious that these are normal human remains, wishful thinkers proclaim them to be the remains of giants. David Hatcher Childress, actually holding the normal-sized skull in his hands, says "these are really the skulls of giant people." I just don't get it. At least when people found mastodon bones in the 1700s they were looking at something that was unexplainable given their knowledge of the natural world. But this isn't that. This is the willful maintenance of a fringe myth that can be easily discarded based on what is sitting right there in front of you. The desire for the "smoking gun" is so strong that not even the most obvious evidence to the contrary can dampen it - when you've made yourself immune to the evidence, you've inoculated yourself to the "truth" you claim to be uncovering. So silly.
If you're mad at yourself for some reason, you can watch this video of M. K. Davis spinning tales about why so many of the skulls from Lovelock Cave appear to be missing. He says that an earlier photo of the cabinet that David Hatcher Childress looked in shows that there used to be more skulls. As pointed out by one of the comments to the video, what Davis is actually looking at is an image that is two photos of the same skulls (in a different arrangement) spliced together. Note that the "shelf" disappears into nothingness on the right side, and the skull on the far right on the top is the same as the second skull from the right on the bottom (you can tell by the missing teeth in the upper and lower jaws). Davis' website makes the same mistake.