Slovakia pulled off the first upset of the tournament in Germany as they scored early and then kept the much-fancied Belgians at bay to win their opening Group E encounter.
“The lesser teams, if you will... the smaller nations, have made progress. And we can cause problems for the more prestigious nations. But there is still a big gap,” said Calzona after the 48th-ranked Slovaks upended No. 3 Belgium.
“We have to work very hard. It's by no means easy because a country like Belgium produces so many footballers.
"But it's also true that the smaller nations are also developing and I realised that in the qualification group, the smaller nations are very well organised, and it's true, they have made progress.”
Limited resources made Monday’s triumph even sweeter for Italian-born Calzona, formerly at Napoli before taking on the Slovakia post two years ago.
“We're a small nation. And it's not easy to find personnel and we have some issues in certain areas of the park,” he said.
But Calzona hailed the win which puts Slovakia on course for the last 16 with matches to come against Ukraine and Romania.
"Our players did their best, they put in the effort and we beat a great opponent. I told the boys that we have to have the mentality that we can be equal to the best.
“It's a huge win, but we have to keep working. We are looking at the next games - we have to score in those games as well to meet our target and advance," Calzona added.
Goal hero Ivan Schranz said it was a deserved victory, even if Belgium had two strikes disallowed, another cleared off the line and dominated play.
“We gave an excellent and disciplined performance. It's one of the goals of a lifetime (to win at the Euros) making the victory all the more beautiful,” he explained.