From the euroncap website:
21. Are large cars safer than small cars?
From the laws of physics, all other things being equal, heavy cars do better than light cars when they are involved in a frontal impact. To avoid mass effects, Euro NCAP tests and compares cars within size categories. The safety of cars in a size category is not a function of mass, but it is dependent upon good design.
22. Can results be compared between groups?
Results should only be compared within the same group. The frontal testing method mirrors a crash between two similar sized cars. Clearly a bigger car has an advantage if it hits a smaller car and Euro NCAP results can’t be used to predict the outcome of such crashes.