I believe I stock usually on the heavy side of the "norm". Somewhere between the "norm" and the JDM but 5 steps away from the "norm" and still 500 steps to go to the JDM.
I also came to believe that the shortage of clear swimming space and lack of rest from all the motion and commotion stresses non-schooling, non-shoaling fish and badly undercuts their quality of life and longevity, I'd say pretty much kills the fish in a JDM tank that should live for 20 years in 20 days to 20 weeks to 20 months. In the case of our tanks, the heavy stock might have been resulting in shortening of lifespans as well, just not as badly as in a JDM tank.
I also don't believe that JDM tanks can be filtered properly to avoid ammonia and nitrite stress, because if this was the goal, the filtration turnover would be so strong that the fish would be blown around and not be able to handle it, and the strong, violent flow would also stress the fish further. JDM density is close to fish farm density, where usually the flow is laminar (sheet flow) and in one direction and often the flow is open ended, that is the water is not circulated through a relatively small filter, it comes in and goes out like a diverted part of a river or a creek or the water is taken from an enormous natural water body, can be a lake or even an aquifer, and then it is returned there directly. Or often fish are raised in enclosures right in huge water bodies, like lakes, rivers, and seas.
Thus, in this regard, I respectfully disagree with Russ. Yes, tank mates can choose who is bullied or eaten and who is not. That's one stressor. Adding the stress of no rest is a different and almost assured way to kill a fish the fastest. Adding underfiltration is just another nail into the proverbial fish coffin, if they existed...
I'd also respectfully disagree with Pacu Mom in one aspect. This is not about the tank size but the stock, the water weight divided by fish weight is a dimensionless measure. One can have too many tetras or guppies in a 5 gal tank just the same. I believe what Pacu Mom is probably saying is that the societal paradigm has been shifting far, far to the left and anyone attempting to keep a "big" fish in anything less than an acre-size pond is seen as committing a cruelty more and more with each generation.