Hm. Sounds like two things going on. You indicated a very large expansion. That sounds like fluid retention expansion more than internal expansion. During
SSJ, this is where a good solid erection is need to get the initial blood volume in for that super hard internal expansion during the
SSJ. When you get that full erection blood volume trapped with your OK grip at the base, that full feeling of engorgement should be there. If you don't feel that full engorgement, either the erection volume was not achieved or the sensation is not felt.
Before the next pumping routine, try a double OK grips tightly around the base of the penis at full erection and slowly slide the grips at full strength toward the glans. Let us know if you feel that sensation like your penis is going to pop like a balloon.
Those who don't have this engorgement feelings can either have over desensitized penile shaft due to over masterbation, or just low nerve responses to touch.