You may have started to develop an internal prolapse, which can cause this. Think of a garden hose that’s bent (as opposed to being straight), you’d need higher pressure for the same output flow. If your descending colon is pressing onto the sigmoid, the passage can become more straining, thus the additional force needed and the smaller stool size. Similar thing can happen if the sigmoid is the one prolapsing and “piling up” on the rectum.
My theory for a lot of people who are into this type of anal games (myself included) who develop a prominent rosebutt and show these signs of internal prolapse is that the sigmoid mesocolon (a fold of peritoneum) has become loose or even partially detached after many sessions which induced some level of mechanical trauma. The reason it hasn’t progressed to an external prolapse is that the descending mesocolon (which is much larger and holds a larger area —the descending colon— thus mechanically more resilient) is still attached firmly enough. Debilitating the sigmoid mesocolon is done much more easily through relatively shorter insertions (only need to reach the sigmoid colon) whereas doing so for the descending mesocolon requires much deeper play. On top of that, to get any meaningful prolapse of the descending colon, some additional give of the transverse mesocolon would be needed, since the descending colon is more or less already in a straight line.
Full disclaimer: I’m not a doctor, so this is what I believe is happening when an internal-only prolapse is developing after reading anatomical texts, but I could have misunderstood or misinterpreted something, so take it for what it is.
Sorry if you find this answer too long or off topic, this is just very interesting to me