Big Mom could have been one of the earlier experiments by the World Government or even Dr. Vegapunk at turning humans into giants.
We haven’t seen Dr. Vegapunk so it’s not far fetched to think that someone who made it possible for inanimate objects to eat devil fruits could also be really old and use some scientific technique to keep himself alive.
Big Mom clearly isn’t normal, right from when she was a child. Dr. Vegapunk could have operated on her while she was in the womb because her parents look normal sized as they dropped her off at Elbaf. Considering how big she was at that age it’s tough to think that she was also born as a huge baby without her mother dying during child birth.
As being born a normal sized baby she must have had to grow at an alarming rate for her to achieve giantism hence the need for large amounts of food. For her to grow into a giant she would need a constant supply of large amounts of food or else she would starve to death with her metabolism making use of all her energy. The dependence on food may have then led Linlin to develop a mental disorder much like drug addicts and also the children at Punk Hazard when they weren’t given their sweets. And which kid doesn’t like food that’s sweet?
Her genetic alteration would have been imperfect which then lead to several side effects; she didn’t become a giant, she was mentally unstable, she had super-human strength, durability, endurance and stamina which made it impossible for her to be stopped whenever she went on a tantrum.
Linlin was then deemed too dangerous and all research to turn humans into giants were banned at the fear of creating more people like Linlin.
*Theory by Swagkid98