Last June, VW said it would invest $5 billion in Rivian as part of a new joint venture that’s focused on developing a new electrical architecture and vehicle software for future models, including subcompact cars, with the first planned for 2027. The investment size has now increased to $5.8 billion.

The big challenge for Rivian, the EV maker known for its innovative electric and software systems, has long been how to reach the next stage of growth. That stage came within reach in June, when the California-based company and Volkswagen announced a joint ...