When you add files to the Lima drive, they are temporary taking space during the transfer: It's called cache.
A cache is a place to store something temporarily. Caching is mostly used to improve application experiences: shorten data access time and reduce latency.
Lima uses cache to improve its performance and make your data accessible much faster.
What is the difference between cache and the offline mode?
When Lima is caching your files, it doesn't make you choose which files: it generally caches the more recent added files, or the more used ones.
The offline mode enables you to keep files that you choose offline: those files will therefore be accessible anytime even without an Internet connection.
Even so your Lima is offline, it had stored files in the cache during transfer: those files are therefore stored in the local memory of your device, before they got cleared up, which make them accessible.