Giant is a relative term, as we agree. The hypothesis is that ice age conditions permitted evolution to potential under conditions that were physiologically challenging, yet biologically nourishing at the cellular level. Megafauna,, compared to present species, were larger and more powerful, but had greater requirements to sustain themselves. This made them vulnerable to extinction, particularly if they lacked robustness or were particularly specialized and vulnerable to a changing environment.
I believe that there were two species of megafauna hominid. The first, a species of near-humans that grew tall, but not robust, due to the ability to use tools, craft clothing, and rely on a cooperative societal structure that both multiplied group effectiveness and forgave individual frailty. I believe that this less robust race became, in post ice age conditions with insufficient oxygenation to maintain cellular health, susceptible to all of the physical maladies that plague exceptionally tall people today, and that they both slowly died out and were absorbed into our species. They may be our mystery ancestor. If accounts of large remains discovered throughout the mound building culture are to be believed, and I believe some of them can, then this race was assimilated into the mound building culture. In contrast, the Si-The-Cah of Paiute legend and archeological discovery, were unable to survive in armed competition with the Paiutes. Other historical mentions include the Susquehanock and the Patagonian "giants".
The second megafauna hominid would be bigfoot. The difference and advantage being that they grew more robust (with the key physiological feature being increased lung volume due to this robustness [depth and breadth of the torso as well as height]), and were able to sustain sufficient oxygenation to sustain cellular health in post ice age oxygen levels, though they are presumably smaller than they once were. Robustness would have been a matter of selection in a species that did not rely heavily on tools, crafted items, or complex societies that permitted the frail to survive and pass on their genes.