So I counted the number of Moments each book had on the old Pottermore. For comparison the page numbers (UK children's edition).

Philospher's Stone	43	223
Chamber of Secrets	45	251
Prisoner of Azkaban	39	468
Goblet of Fire	39	636
Order of the Phoenix	12	766
Half-Blood Prince	12	607
Deathly Hallows	14	607