Questions about the physiology of anal sex

I (M59) have started using a fairly large glass dildo for solo anal sex. My toys are 10 and 11 inches long, with insertable lengths of 7.5 and 8.25 inches. I’ve noticed that when I play, the dildo may only be inserted halfway at first. However, after a while, suddenly, something relaxes or moves, and the whole toy slides in easily with almost no resistance. (It’s very pleasurable.) Does anyone know what this is about? What exactly is relaxation? How big a hard dildo can one actually use before entering the danger zone? Am I getting there yet? I try to be very gentle and use lots of lube. However, I do worry that I may be doing myself some kind of harm. Any insights?

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