Thames Reach complained to the Portman Group, saying that the cans contain 4.5 units of alcohol and so encouraged drinkers to exceed their daily guidelines in a single serving. Men should not regularly drink more than four units a day, government health advisors say.
But Portman's independent complaints panel disagreed that the can size automatically makes people drink more than they should, and said the producers were not breaching its code of practice.
The panel said the government issued sensible drinking advice "as guidelines rather than strict limits" and also said it was wrong to single out strong beers when ciders and wine were also high in alcohol and sold in containers that were not easily resealable.
Portman chief executive David Poley said: "While panel members appreciated Thames Reach's concerns, they decided that restricting container size would be inappropriate and liable to lead to inconsistencies."
But the panel agreed that the packaging of Kestrel Super was too dominated by references to its strength and ordered Wells & Young's to make changes.
Thames Reach chief executive Jeremy Swain said Portman had been "completely discredited" by the panel's decision and called on brewers to restrict can sizes to 44cl, as Inbev has done with Tennent's Super.
He added: "In an ideal world we would like to see the product banned. But Inbev UK has to be applauded for taking the lead and setting an example to others by reducing the can size and boosting the price. I would urge Carlsberg, which has so far been depressingly impervious to our campaign, to respond to this brave step by reducing the can size of Special Brew and Skol Super at the earliest opportunity."
Wells & Young's has agreed to make changes to its packaging on Kestrel Super but expressed disappointment with the decision.
"Kestrel Super, in its present packaging design, has been on sale for many years," a spokesman said. "It was always believed that the images and text used within the packaging design were in line with Portman Group guidelines."