This is an interesting situation you're experiencing, and there could be several reasons as to why this is happening:
1. **Metadata**: When files are copied to a different location, there are some additional information that gets attached, such as file ownership data, timestamps etc. This additional metadata could increase the size of the files subtly.
2. **Compression**: The original files on your C: drive may be compressed and when copied to the external drive, they could get decompressed, thus taking more space.
3. **File System Difference**: If your C: drive is using a version of NTFS and your external drive is using FAT32 or exFAT, there could be size differences due to the way each system handles file storage. In simple terms, the size shown by Explorer is based on the assumption that the file will be stored on an NTFS system (which probably is the case for your "Documents" folder on C: drive), and if when actually copied to a FAT32 or exFAT drive the size might get larger due to the greater block size that FAT32 or exFAT uses.
4. **Hidden Files**: Ensure that Windows is set to show hidden files and folders, because it's possible that the original folder had hidden files, which have now been unhidden when copied to the external drive. Identifying hidden files and folders can be done through the "View" tab in "File Explorer Options", and by ticking the "Hidden items" checkbox.
One way to find what's different about the 30MB additional space is to use a tool like WinDirStat or TreeSize to get a detailed, visual representation of where all your storage is going.
Remember to always safely eject your external hard drive to prevent corruption or loss of data.