well, the lining of the anus and rectum is quite thin so it can tear quite easily when something is inserted beyond a specific point. This is why sometimes blood comes out when u have problems pooping. This is called anal fissure, you can read more about it online. So, unlike the vagina, the anus isn’t as stretchy and the skin inside is much weaker so it tears easily.
Moreover, you can try to avoid this by using better lube that helps in sex.
