This year will be the third in succession that the economies of the EU 15 have in aggregate outperformed the Euro 12.
So if the EU 15 is the Euro 12 with the addition of Sweden, Denmark and the UK, then won't the EU 15 always outperform the Euro 12? Or maybe they are talking about the fact that the three countries that haven't yet adopted the Euro are outperfoming the 12 countries that have.
And I believe the metric is growth, not size - i.e. the EU 15 has grown, say, 1.5% per year, while the euro 12 has grown 1.3%. You see that when we're discussing growth rate, it's possible for the Euro 12 to outperform the EU 15 if the three noneuro countries have lower or negative net growth.