The new GoPro session camera weighs only 74g or 2.6 ounces. Initially this does seem all that much lighter than the original 88g weight of the likes of the Hero 3 series. However a crucial factor is the case that is required for mounting which for the gopro Hero 3 increases the overall weight to 152g. Interestingly on the gopro site there is only one weight listed for the GoPro Session rather than the camera and camera housed for the other cameras. If this is the case then it will mean that there is a saving of up to 78g a healthy 52%.
So when would this make a difference to drones? I think that if you are making a drone specifically for the purpose of filming the Hero 4 will be better suited because of the quality of image. However with the racing drones of the 250 class the GoPro Session is ideal shedding weight which makes a massive difference.