Question: How could the Erumpent the size of a whale escape out of Newt's case without Newt around, without his help (magic)? Can Erumpents shrink themselves like Occamies?
Answer: This might actually be a valid mistake. The exit of the suitcase is depicted as being inside a hut, at the ceiling. A savanna creature like that would not even consider that to be a route it might be able to take, because that requires (sorry for the pun) out-of-the-box thinking.
Perhaps that would be true for a non-magical creature; however, there are numerous references in the Wizarding World canon to magical creatures have human-like cognitive abilities (Kneazles are one prominent example), so perhaps Erumpents fall into the same category.
I agree under normal circumstances the creature wouldn't try to escape. However, as Newt pointed out, this creature was in mating season and was looking for a partner; which is why she went to the zoo as there were the closest creatures she sensed to herself. Animals going out of their natural habitat when they sense a potential mating partner is a natural survival instinct real animals do. As real animals do this, it makes sense this creature would do the same thing as a real animal.
As said elsewhere, the exit is depicted as in the ceiling. Erumpets aren't displaying any intelligence beyond that of a bull or rhinoceros. Climbing out through a roof hatch is not in their behavioral patterns, mating season or not. A bull won't climb a ladder, no matter how many cows are on the roof.
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 ★