Tumescence is the quality or state of being tumescent or swollen. Tumescence usually refers to the normal engorgement with blood (vascular congestion) of the erectile tissues, marking sexual excitation, and possible readiness for sexual activity.
This isn't just an issue with males (creating erections/boners) but also females.
The corpora cavernosum don't just inflate in males (externally: in their dick) but also in women too (internally: along the walls of their vagina) and women also have vestibular bulbs in between their vaginas and their corpora.
The swelling of these when women are horny for Chad makes women's pussies tigher and makes average dicks feel like large dicks and small dicks feel like average dicks.
This means that Chad gets to fuck tighter pussies (and his dick will feel larger) because women's horniness for Chad allows them to emulate virginal tightness.