Question about glans size during natural erection

Hiya, I’ve noticed this often when I wake up in the morning or when I get aroused with a prostitute without touching my dick, my glans doesn’t achieve the same large side it does when I am jelqing. My shaft is thick, maybe more so than usual, but my glans is often small.It seems to take some handling (if I’m with a prostitute) before it gets to it’s proper size. I’ve got a Chinese vacuum hanging cup in medium size and even when semi my glans struggles to fit into the medium cup, so anyone with a vacuum hanger would know that it gets fairly large if they use that as a reference. But during a “natural erection” it seems to not achieve it’s full size, as if the blood isn’t getting all the way to the end.