Explosions in space movies aren't actually that unrealistic. If you're talking SW, realistically, those ships would use antimatter propulsion. If containment fails, boom. If anything, those explosions are too small.

In case of low-tech / current-tech sci-fi - as you've noted there's no fire in space, so in order to burn their engines ship need to carry their own oxygen (or other oxidiser) - if ship is damaged and they mix boom.