I have to admit, when I first saw this pic i thought it was a cheap Claire's imitation tiara than the real thing. Not that it looks cheap on Amalia, I think the lighting takes some colour out of the diamonds, and I'm surprised at how narrow the tiara is, when Max wears it, it always looks wider imo
I think so that it's due to the lighting. As for the size of the tiara, I think the base part of the tiara is adjustable and can be worn bit narrow or wider to fit the size of one's head.
We can see side by side picture of the tiara worn by Amalia and by Maxima in this picture