The girth of a toy and the girth I’d your partners penis can be misleading. Materials like glass, and even many silicon or rubber toys don’t have much if any give to them, your partners penis, even when hard, is relatively soft and flexible.
If the pain you describe is a sharp sort of ouch pain as the toy reaches full girth then that’s basically just stretching your sphincters. You have two exterior sphincters so to speak, first the external one that is your anus, and another a inch or two inside, the inner one is often the more uncomfortable one.
Two things I think can help other than using a lot of lube. First get a lube launcher and put the lube inside. Second, get a ribbed dildo of appropriate girth, something like the Lovehoney beaded glass dildo or the doc Johnson’s B-7 butt plug(this is a bit larger). Play solo a bit until you can comfortable thrust it in and out over the bumps, practice pushing against the toy almost like your going to the loo (this relaxes your sphincter muscles). Before long you will have some muscle memory and will be able to accommodate toys without pain.
Finally it is worth saying, even if you are experienced with anal and can take a decent girth toy or penis, if it is another person inserting things, they will need to warm you up beforehand with fingers and smaller toys, your body is less able to anticipate the insertion and from my own experience, toys that would not otherwise be any issue can hurt a bit when the muscles are not ready.