All over the world, there has been fossils found of giant crocodiles, trees, dragonflies, tigers, lions, etc. In fact even recently, with the Global Warming going on, there has been incidences of very giant squids being fond - as giant as block where a double-decker buss would look small.
Giant Humans are also mentioned here in this site: Giant's Site
When I was a child, my father would say such things and I would think it was just a story. Then in Mahabharata and Ramayana stories, such Giants are mentioned, as they are mentioned in Bible. Another feature of these Giants was that they would live for long years - like the age of 70 was youth and time to marry - like in Mahabharata, and it is said Ram ruled for more than 1000 years.
In Bitthud near IIT Kanpur, in India, which is mentioned in Ramayana, next to Rive Ganga or Ganges, there is a temple, where two giant Arrow Heads of some Brass kind of Metal, of more than Throw put Ball Size. The priest and his son said said they were found in the river bed, and they showed the displayed certification from the UP State Govt Archaeological Survey. I asked if could purchase them, and even though they were very poor, the son was upset, and they said they were price-less. They said that the dating showed it was from Ramayana Period.
It is a well known fact that Indians have had no sense of History or History keeping, though on contradiction, the vast amount of literature of Vedas, Upanishads, Shruti, Smriti and Itihass show a uncanny sense of duty to preserve the past, but this seem to be not displayed by Indian Historians (I have found more information as a hobbyist and part time maverick) of last 1200 years the least.
One reason is that India was ruled for last 1200 years, by outsiders, and Historians are only paid to repeat what the masters want - thus killing the originality. And then one needs the wealth of money to feed the researchers, which has been found lacking in the said period. This is a job of those who do not have to worry about day to day struggles and are supported by their Govt.
The only good work recently done has been by the ASI in finding the Dwarka site as mentioned in Mahabharata.
There are many theories:
a. Increased Oxygen Supply Percentage Wise.
b. More Atmospheric Pressure increasing Oxygen's Reach.
b. More Available Natural Resources for Food.
c. Less Gravity - further away from Sun or Milk Way's Black Hole.
d. Gravitational Shielding because of Some Planet "X" (the Shattered Solar Asteroid Belt?)
e. Alien Theory
f. Relativity of Time - Increased Solar Days giving more sun light for growth of plants and longer "wake" and "sleep" cycle of living beings - where the swings are bigger and one needs a bigger body to sustain in terms of metabolism (sounds weired one one thinks of warm blooded animals where smaller body would help regulate the temperature, that is via Autonomous Nervous System - but here a larger Biomass System would be more insulated to fluctuations and take long to show any changes - like a specific heat problem, and both a relevant discussions in terms of the nature of "optimization problems" where a "Hill Pattern" is often noticed).