Jeff Posnick schreibt in Workbox A common source of unexpectedly high quota usage is due to runtime caching of opaque responses, which is to say, cross-origin responses to requests made without CORS enabled. Browsers automatically inflate the quota impact of those opaque responses as a security consideration. In Chrome, for instance, even an opaque response of a few kilobytes will end up contributing around 7 megabytes towards your quota usage.
CSP nonce values can help you securely run inline content on you site. But it can be hard to get it working with Service Workers... until now.