The engines may be smaller but there's also much less wind resistance because of the flat design.
For me, a Mavic would mean getting hundreds of shots I would not get at all, since I wouldn't bring a big drone. I'm using the Phantom 3 Pro right now, and while I wish the video quality was better, it's pretty good for HD in most situations with even lighting (morning/evening). The Phantom 4 camera is really no different (except for MORE sharpening in-camera, not desirable).
I would of course like a step UP in video quality, but if the Mavic offers something close to the P3P/P4 the size and portability will most likely make up for it. I guess you can't have it all... I would love to use an Inspire RAW but it's just a complete hassle to bring when you travel (planes, other countries, hikes, etc.).
Being able to put it in your pocket is a pretty big deal. Carrying the P3P along with all other camera equipment up a mountain gets old quickly and means leaving the drone at home...