That's very likely because of photos and videos you've sent and received. Open Settings>General>Usage and you may find your Messages app occupies a few GB of space. The capacity to archive messages in a usable format is also useful if you need to create space on your device. It's not perfect - it doesn't recognize who sent you a message, but files them by phone number, however you can preview individual messages before you archive them off your device. When you rescue your messages PhoneRescue saves them as HTML files, which you can read in your browser. This is particularly useful if you need to archive message threads for legal or sentimental reasons.
I think PhoneRescue's capacity to archive your iMessage/SMS messages is really useful, as that's something I've been looking for. You should always backup your device to iTunes/iCloud before using this utility, in case something goes wrong. You can even choose to rescue specific data - photos, contacts, calendar, messages, for example. There are a few foibles - I noticed the software sometimes stalled, but tried again and it managed the job.
The utility worked well in both cases, making recovey possible through a simple interface. I've tested PhoneRescue both with my own phone and to recover data from a faulty device.
