This has been changed back in Hype v4.0.2 – motion paths are now included as part of thin runtime. Thus the thin runtime grew in size 2.0 KB from 4.0.1 to 4.0.2 for this feature.
No, as if all features were pulled into the thin runtime it would not be thin anymore. For example, including Vector Shape support alone would add 9.8 KB.
The full list is of features that would trigger using the full runtime (as of v4.0.2) is:
Physics
Vector Shapes
Motion Paths
Audio
Swap Transition
Page Turn Transition
Control Element Position when in a flexible scene
IE 6-9 suppprt (determined when visiting the page, not statically)
Note that both physics and waypoints usage will also trigger additional files (HYPE-NNN.physics.min.js and HYPE-NNN.waypoints.min.js) being loaded. Physics still requires the full runtime because that contains the integration code between the Hype runtime and physics file, which is essentially just matter.js.
The opposite is the intention - we do a lot of work to try to keep the runtime as small as possible, hence these changes. Over the course of many years of minification work there's not a lot of optimization left to do other than removing features that we don't think would be used in digital ads. We reverted on Motion Paths after hearing feedback on how commonly used this was. It is all a delicate balance between features, CDN-ability, and filesize.