Queen Fabiola also wore the Spanish tiara in its coronet version at least twice in 1966, during the State visits paid to Belgium by Queen Elizabeth II of Great-Britain and King Gustaf VI Adolf of Sweden:
Queen Fabiola had begun to wear the wreath version of the tiara as soon as May 1961 (State visit to France), i.e. less than six months after her wedding:
So, between at least 1961 and 1966 (and possibly a few years later, as I have a few pictures of which I don't know the date), Queen Fabiola wore alternatively the two versions of her Spanish tiara. Strange that, after the late 1960s, she always opted for the wreath version.
Like Maria-Olivia, I am not really convinced by the argument of the size of the tiara being the reason for Fabiola to avoid the coronet version. In its coronet version, the tiara is large, indeed, but not overwhelming, and not much larger than the Nine-Province Tiara or the Spanish tiara in its wreath version.
And I agree with Maria-Olivia that the base of the coronet version of the Spanish tiara could make a very nice "smaller" tiara - maybe with the large leaves replaced by the pear-shaped aquamarines visible on the picture below, on the right, hanging from Queen Fabiola's necklace: