Corpus spongiosum
The corpus spongiosum is the third chamber. It is a space the fills with blood, like the larger corpora cavernosa, but it is smaller and the tissue surrounding it is not as tough as the tunica albuginea which covers the corpora cavernosa. The urethra is enclosed within the corpus spongiosum and the glans penis is the expanded end of this tube.
This drawing shows the relationship of the three chambers. It is a view from the bottom. Notice that at the proximal end (close to the body) of the corpus spongiosum expands into a bulb. You can feel the bulb of the penis if you press in just under your scrotum. Kegel’s exercise compresses the bulb and, when done erect, forces some blood through the corpus spongiosum to the glans, making it expand. If you press on the bulb with your fingers when erect you can accomplish the same glans expansion, often to a greater extent than by doing a Kegel.
Since the corpus spongiosum is not rigid like the corpora cavernosa, it has little to do with erection and penetration except to get blood to the glans. So any “work” done on it is really glans work. It might be possible to expand the glans with manual pressure on the bulb to an extent that it might grow slightly. But I imagine it would take a lot of concentrated work to do that.
One caution. If you’re going to be pressing on your perineum with your fingers, just remember that the perineal nerve is there. If you’ve ridden a bicycle with a normal bicycle seat for any length of time you know that constant pressure on the perineal nerve can make your dick “go to sleep.” Just be careful.