Sand weighs ~100 lbs per cubic ft.
37 tons = 74000 lbs or 740 cubic ft
3 inches of depth gets you 4 sq ft of coverage
740*4 gets you 2960 sq. feet.
The city says the park is 14,000 sq feet. [1]
In the video you can see they avoided spreading it in a lot of areas because they didn't have nearly enough!
Conclusion: Sand is heavy
[1] https://www.san-clemente.org/Home/Components/FacilityDirecto...