Jeff Posnick writes, wrt to 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.
Service Workers are an amazing and integral part of the web ecosystem, but there are still quite a few gotchas - and this is one of them that can bite you if you don't know this ahead of time.
It's great to see tools like Workbox being able to handle this and inform you so that you know what happens.