In many (former) kingdoms there are crowns with 8 arches. The Imperial State Crown, created in 1937 when George VI was still Emperor of India, has 4 arches. I suspect Queen Mary's crown was modified to match the 4 arches of St Edwards Crown and the Imperial State Crown plus it reduced the weight somewhat. Still, I think the proportions of this crown are not very flattering. A further adjustment is needed.
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I read somewhere (can't remember where) that the extra arches were removed because 8 arches are for an imperial crown (ie: for an empire), and the UK is not an empire anymore. But I'm sure making it lighter was a welcome bonus.