Thanks for your responses. Some here claimed, that the response to the 3DS was not as good as in Japan, so it makes sense that the XL is more popular over there. But there is a critical point here: I was talking about relative number (percent), not absolute numbers. So even on the lower level of sales, that Europe had, it did a waaayy smaller relative increase than the increase in sales in Japan. So the gap even growed.
To make this clearer: In the week ending 21 July, the sales in Europe were 28,118, in Japan it was 43,291. So the sales in Europe were 65% of what was sold in Japan. The week ending 28th July (XL-release) the numbers were 71,708 in Europe and 239,273 in Japan. So in Europe was only sold 30% of what was sold in Japan. The discrepancy got even worse.
Same thing with smaller game-library in Europe, as Rol pointed out. That explains the lower level of sales. But why is the relative interest in the new model so much smaller than in Japan.
A more valid point is, that Japan had a day more of sales. That may account for a part. But as first-day-sales are often very important, I don't believe it explains the whole discrepancy.
Also the parallel launch of NSMB2 in Japan was pointed out. That's seems plausible. The game-release-boost and the new-model-boost got combined in Japan. If that's the case, we should see a good effect on sales, if NSMB2 comes to Europe.