Santa is one smart cookie.
He knows that every year millions of children will ask him for coloured plastic building blocks.
He doesn't waste time and effort by tasking the elves to produce the required numbers of said plastic building blocks.
No, he contracts out.
Children receiving Lego with a value of more than £50 (a target easily achieved and surpassed without effort, if parents are not careful) at Christmas would be entitled to visit the Lego store in the New Year and fill a box of Lego for free.
For size, think small American-style Chinese take-away carton.
For filling, think mummy with eleven years of military Logistics experience.