I always thought laws on pepper spray were silly.
I can still recall when there were almost no laws on it in most states.
They didn't have any restrictions on the strength, concentration, or amount.
Pepper spray was released widespread in 1991, and there was very few restrictions most places until the late 90s.
4oz was the standard size before they started making smaller key chain versions.
It is the size mailmen have been carrying for as long as they had some form of chemical spray. Mace, CS, and related products before pepper spray replaced them.
The bear sprays at 16 oz commercially available for self defense against animals are where they get large.
These are also about the same size as the typical Law Enforcement foggers.
(16 oz bear spray)
(17.4 oz LEO fogger)
Some civilian sprays are also sold for RV and home protection in similar sizes:
(16 oz spray)
There is actually many sizes available. They even have huge fire extinguisher containers full of it. They use such things in law enforcement and especially corrections:
(46 oz spray)
(25 and 50 oz tanks)
I have also made extract from my own habanero peppers stronger than anything sold and concentrated it, for a food additive. A couple drops goes a long way. Food additive OC is actually sold at the commercial level which is much stronger than any spray, and it is where most of the OC spray manufacturers obtain theirs.
They then dilute it and add a carrier.
Pepper spray can be dangerous. It can harm eyes even causing permanent damage to the cornea through inflammation and irritation in some cases if not promptly treated. If any sizable quantity gets into the lungs it can also inflame the lungs, preventing breathing. These are just risks for someone sprayed, never mind if you add choke holds or other restraint techniques in combination.
However such sprays are widely used in prisons, and inmates continue to fight and stab each other even drenched in amounts and concentrations far greater than anything typically carried on a keychain or in a pocket. The only thing it can be reliably expected to do is blind and allow for escape.