really I think the most important is direction of push and the sequence. bones stay on the way of plug when you stetch to the wrong side. do not push the most common way, most of plugs won't fit at first, but when you stretch your inner muscles it goes well
i see what your saying and its true to a point. but when your toy is large enough that it stops on the the bone you can feel solid contact on your bones they must spread a little to let that toy through. when i got toys that where to wide to fit through i started working them through by rocking back and fourth once i got one side of the toy pass the bone i would rock to the other side and it would slide in (very tight fit) over time i can bottom on the bones and keep pressure on them and once i could relax (my inner muscle) it would pass through.....It all takes time and a LOT of lube..