public readonly ulong packedSize { get; }

This is the size prior to any compression. The actual size on disk can be smaller when the file is stored inside a Unity Archive (e.g. in the case of a AssetBundle, or a Player built with BuildOptions.CompressWithLz4HC).

Note: there can be extra padding bytes inserted between Packed Assets, added for alignment purpose. Because of this, the offset + packedSize may be slightly smaller than the offset of the next element in the PackedAsset array.

Additional Resources: _contents