After the Battle of Hogwarts, a lot more people were admitted into the school, from multiple countries and all different ages. This was because the whole worldwide magical community heard about the battle and it brought new attention to the school. And who doesn't want to learn magic at a place where the students triumphed in such a legendary battle? Even people who had been hiding away and had never been to a Wizarding school enrolled in Hogwarts. So many people, in fact, that the new staff at Hogwarts had to enchant and enlarge the castle on the inside to accommodate everyone. And that's what HiH is. This is why some students can cast the Patronus Charm or are Animagi even as First-Years. It also explains why some people know more spells than others. Also, the influx of Muggle-born and Half-blood students is why Hogwarts has adapted to the times and started integrating Muggle technology (e-owls, etc.) into its curriculum. Because even with magical enlargement, the castle still can't fit EVERYONE.