Actually it makes perfect sense.
If Mazda would be able to spend more, it could also do the testing, add padding, more crumble zones, air bags, etc., to also get a better rating.
More expensive cars will probably always be safer than cheap ones.
It has always been that way, regardless of size.

And example was the Bricklen.
It was smaller, but safer than the Lincoln.
It just was expensive.

Bricklin SV-1 - Wikipedia, the free encyclopedia