Perhaps as the wedding day drew closer the Queen had some second thoughts about giving Mary an heirloom Tiara, from a historical point of view it was probably cheaper to buy one at auction to give to Mary ... just in case. Whereas Queen Igrid's ruby-set belongs to the State and must be given back so Christian's bride may have them
I heard that they bought the tiara in auction.
I think that the queen (and prince consort) had learned from what has happened with joachim marriage then they don't want to loose any other family heirloom tiara, but to buy a new "bigger" tiara was not in their list becaused the cost of the wedding itself already very costly (and as I know so far DRF is not one of the richest royal family).
If I'm not mistaken Ingrid tiara is a private collection (not belongs to state), the tiara was given to Frederik to be worn by his wife. I am not sure if Mary is allowed to take the tiara with her if the marriage doesn't work (I more think that she is not)
. for the small tiara, it belongs to Mary and she can take it with her if they divorce.