Build in grip not necessary add bulk to the camera, it is just a vertical grip that makes shooting a lot more comfortable under different situation, of course Pentax must make the grip small enough. The grip can be simply a tiny add on to the body without electronics, but the on camera battery must last long enough. The main reason I add the grip is to add some water protection to the camera, coz my last K3 ruined in the rain that water entered from the battery door (Pentax replaced the camera FOC), also you don't want to change battery in the middle of the rain.
Let's go back in history take a look at the Canon T90 :
Canon T90 - Wikipedia
It was the most innovative film camera at the time, it got a not so comfortable vertical grip built in, with an add on release bottom option, it takes 4AA cell and able to shot at 4.5fps that's uncommon at the time. In today's technology we could fill up the body with electronics and LIon battery without the need of an add on grip. We may not need super high speed stills and 4K video, but we will want long battery life and ergonomic design, the reason that design does not exist in Pentax is marketing reason !