The Magizoologist in Harry Potter is a man called Newton Scamander. The spell that NewtScamander can use to make his case big enough is Extension CharmorCapaciousextremis.
Capacious extremis when translated means capacity and spacious. This spell helps wizard or witch to alter small enclosures such as like bags and making all kinds of extra invisible space within them.
The Magizoologist Newton Scamander was known to have a magical suitcase that was bewitched with an UndetectableExtension Charm.
He placed a lot of magical beasts while inside it as he traveled.
