As your link says "Some research has shown that increasing the severity of a punishment does not have much effect on crime, while increasing the certainty of punishment does have a deterrent effect.[11] "Clearly, enhancing the severity of punishment will have little impact on people who do not believe they will be apprehended for their actions."
Consider, for example, the knowledge that an action will probably result in your early and painful death. This is a very severe result, and you might expect it to have more deterrent effect than a temporary discomfort.
As you will have observed however, and as behavioural science confirms
"If you smoke cigarettes, then, in forty years or so, you might get emphysema, lung cancer, chronic bronchitis and be unable to move from your chair without an oxygen cylinder, and you will probably die long before your time" is very ineffective in preventing the tobacco companies from recruiting new young addicts to replace the old ones who are killed off.
But "touch that iron with your finger and you will get a small burn" deters people from touching the iron. It is certain, personal, and immediate.
"If you murder someone, and you are caught, you might go to prison for twenty years." If that fails to deter you, then it appears that a long prison sentence is not much of a deterrent for you. Although possibly you will decide to kill any witnesses.
Personally, I am undeterred from committing murder by the severity of the sentence. I murder just as many people as I feel like. Which is none.
The assumption that increasing the severity of punishment reduces crime is mistaken.