Instagram have added pinch to zoom on photos and videos in feed, on profiles and on Explore.

instagram

Over the past six months, you’ve seen many exciting changes come to Instagram, from a new icon to longer videos and even a new way to share everyday moments with Instagram Stories. And as things change, we’re still focused on improving the core parts of Instagram.
PINCH and ZOOM!!
Zoom is available today for Instagram on iOS.
Zoom will be available on Android in the coming weeks.