Europe's governing body opted eight years ago to expand the competition from 16 to 24 teams. The additional eight places allowed Albania, Iceland, Northern Ireland, Slovakia and Wales to qualify for the first time.
Some of the newcomers have been particularly successful, including Northern Ireland, who reached the last 16, and quarter-finalists Iceland. Wales have reached the semi-finals where they take on Portugal on Wednesday in Lyon.
"People were anxious the format might not work but it has worked," Kallen told reporters. "We have seen two teams (Iceland and Wales) going further than anybody believed they would and that's positive."
Some people have criticised the new format, saying it has not increased the excitement of the tournament and suggesting that Wales and Northern Ireland would have qualified for a 16-team tournament.
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But Kallen said the less glamorous teams have been supported by some of the most passionate fans in the tournament.
"This has opened new areas for football," he said. "I think there will be a boom in those countries and I think we well see more kids there playing football."
The 24-team format will continue at the 2020 finals and Kallen suggested UEFA might stick to it after that.
"We don't know yet but at the moment it's the format we're looking at because it's been very positive," he said.
UEFA has no problems with the organisation or security at the tournament which has been largely incident-free since violent clashes marred an early game between England and Russia in Marseille.
The governing body has faced criticism, however, for the way it allocated tickets to the football associations. Ireland manager Martin O'Neill called the system unfair.
"In the past we always had trouble to sell tickets because there is not much time from the moment you qualify until the match starts," Kallen said.
"We have a new ticketing system in place which will give us much more flexibility for the future," he added without elaborating. "We take the criticism we get into consideration."
Ticketing, overall, was a success with most matches sold out.
(Editing by Adrian Warner.)
