In iOS 11, the photos and videos you take will no longer be encoded in the compatible formats you’re used to. Instead of JPG for images and MPEG-4 for video, they’ll use High Efficiency Image Format (HEIF) and High Efficiency Video Format (HEVC), respectively. Files in these newer formats are about half the size of JPG and MP4 files of similar quality.
Source: How-To Geek
