Both the iPhone 6s and 6s Plus are set to record in 1080p at 30 frames per second and for slo-mo, 720p at 240 frames per second by default. You can change these by heading to Settings -> Photo & Camera and scrolling down to "Record Video" and "Record Slo-mo." Tap on either of these to change them. As you can see in the notes in the two bottom images, your resolution and frame rate can greatly affect the size of your recordings.
How do I change the video recording resolution on my iPhone 6s?