Black pine is not normally grafted. Usually just as quick to grow from seed so I guess this was some named cultivar that's been grafted onto another pine root stock. The lower section looks quite different to the top part so it is probable the root stock is some other pine species which may explain the differential growth rates.
There are more options than just plant it deeper.
Black pine usually respond to layering so it should be possible to develop a new, and probably much better, root system right at the swelling. There is a small risk to layering pines so need to decide how valuable the tree is currently and benefit/risk.
Another option is to scar the trunk where it's thinner. Scarring produces accelerated growth as the wounds heal so can increase thickening locally. I have not used that technique on pines. The trees I have used it with took a long time to respond so it could take some years to reverse the problem. There may still be the same issue of the stock growing slower so, even if you manage to rectify the problem now, it may recur in future as the 2 sections continue to thicken at different rates. Again, there's some risk that it may not work as intended.
A more complicated solution would be to approach graft seedlings around the base to start a new root system. Similar to the layer option but probably less risky. Would need to be proficient with grafting to pull that one off though.