OK, simplified reason IMO.
THC is fat soluble.
So, it is likely the 100lb female burns fat to fuel her body, instead of carbs, otherwise she would likely not weigh 100 lbs.
Now the 350lb guy is likely fueling his body body with way to many carbs and not enough fat, which is likely the reason he weighs 350 lbs.
So I surmise the woman has used up a lot of the THC burning fat before it has time to metabolize. Just a wild guess.
