iPhone Music app “Documents & Data” is too big after turning on “Sync Library”

I have an iPhone 6s using iOS 15.5 and I turned on “Sync Library”, so I can access all my MacBook mp3 library from my phone. Can’t remember exactly when it started, but “Documents & Data” storage of iPhone Music app is too big (currently 8.15 GB). If I turn off “Sync Library”, then “Documents & Data” storage decrease to around 2.50 GB.

I’m not sure if the increase in size after turning on sync library is a common issue for iPhone users or is it because the greater the mp3 library in a MacBook (mine has 440 GB), then much more storage for “Documents & Data”. Anyone with similar situation?

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