We grabbed a GoPro Hero3+ for comparison; here's what we found:

Same weight: 74 g.

Almost the same ports. Both feature microSD, but the Session moves up to micro-USB instead of mini-USB, and drops the accessory port of traditional GoPros.

The Session also hides its ports under a water-sealed hinged door, while the 3+ keeps its ports under a pop-out door that we took out and lost.

Very similar volume! By our rough understanding of geometry, both the Hero3+ and the Session are just over 50 cubic centimeters in volume.

So is it actually fair to call the Session smaller? Yes. Waterproofing the camera lets users ditch the bulky case, which more than triples the volume of traditional GoPros.