Continuity mistake: After eating their meal they're sitting at the table talking with empty plates in front of them. As they continue to talk all the plates and flatware disappear.
Continuity mistake: During dinner at the Goldsteins', the dishes are empty from one shot to another.
Chosen answer: Erumpents cannot change their size, however the suitcase is itself enchanted to allow objects of any size to enter and exit its magically expanded interior. Newt's presence is not required for this.
Phixius ★