I think it is based on the idea that staying as close as possible to the infectious dose leads to a better outcome/less severe progression. The infectious dose being "the average number of viral particles needed to establish an infection"[1].
There are also rumours (and I stress, rumours) that healthcare workers are getting more severe disease due to being infected with a higher infectious load. This would also fit with this theory.
So the studies show that there is no correlation between viral load and severity of symptoms. One study showed there was a correlation between amount of virus in the nose and severity of symptoms.
No studies have looked at various levels of initial dose (does initial dose correlates to viral load?) I imagine such a thing is very hard to measure.
Can’t remember right now where I heard, it was in a podcast. Will return with the information if/when I find it.
An epidemiologist said that there is correlation between the viral load and the severity of the resulting illness in studies on mice. He also said something that surprised me, namely that the mechanism involved is unknown. It seems fairly obvious that it has to do with the exponential growth of the pathogen inside the body.