They certainly wouldn't add more storage than they need. But "need" is a relative term. If having a bit more storage means you aren't bricking customer cars requiring expensive service, then you "need" the storage.

It's more likely that the update process itself is a bit more complicated than e.g. updating a phone, and unexpected errors in specific processes make it harder to guarantee the safety of the device. For example if an update fails because one of the devices failed to flash, one may not be able to easily re-flash it and it may indicate a hardware fault.