Multimedia files changed in size for no apparent reason

Question

I synchronize my data between my laptop's SSD and a pendrive. A few days ago my sync software (FreeFileSync) reported that hundreds of files has changed in size, despite that i haven't done anything with them.

The changed files are generally multimedia - *.jpg, *.png and a few *.mp3's. While binary comparison confirms that the files are, in fact different, the actual images appear to be identical (Beyond Compare confirms that).

The difference in size is varying, but there are some patterns: mp3s are ~4 KB bigger on my laptop, jpgs are quite consistently ~6.5-7 KB bigger on my laptop. On the other hand, pngs differ wildly: the differences go from a few dozen bytes to over 100KB (and there's no fixed ratio between filesize and difference between files), and in most cases the pendrive ones are bigger.

I though that maybe there is just a difference in metadata, and indeed, exif information was slightly different, but the difference between exif was smaller than the difference between files.

I've uploaded a few of these files to virustotal.com, and it didn't detect any virus. What can this be and what should i do? Should i simply overwrite bigger files with smaller ones? Something like this has never happened to me before.

I'm using Ubuntu 12.04; my laptop copy of the data is on an NTFS partition, and the pendrive is FAT32 - but i don't think that the difference of the default file cluster allocation size matters here, because i would have noticed it before.

Answer 1

You metadata has been changed. When you compare the metadata using this online metadata viewer, of the collet files, you find that the larger file has more metadata (aprox 4.2k) which accounts for the bulk of the file size difference. The bigger file has added padding, and the thumbnail size is different, along with there simply being more exif data available to view. Point is that something has modified your exif data, probably a legit program either doings things someone told it to, like save, or someone didn't know they were telling it to (rotating in microsoft picture viewer saves even though you simply wanted to rotate. I'm sure other programs have similar pitfalls)

There is still an additional 2K +/- of metadata unaccounted for, but I would bet with some research you could figure out what it is. Identifying what program did what would be difficult in hindsight, but in answer to your question, I would say it is safe to overwrite the larger files with the smaller ones unless you think you will need the extra metadata in the future. Although with the simple addition of padding found, I would bet that the extra space might come back if whatever happened before, happens again.

There is metadata with so many multimedia files that I would assume this is the case with all the file types you listed.