This is a tough area to get into because of the nature of Kydex, it can be uforgiving when you try to spot treat it.
I would start by examining the area's where it appears to be the tightest.
Take the knife out of the sheath and use a mini torch or lighter to heat the area. Don't hold the flame too close or let it touch the material or you could ruin it.
A heat gun works god but if you're not
very careful you'll loosen the whole sheath beyond useability. If you do try a heat gun I'd reccomend a reducer which cuts the diameter of the gun down to about 1/4".
Heat the spot for about 20 seconds and touch it to see if it is pliable, if it is then resheath the knife, if not apply the heat for another 5-10 seconds and repeat until the fit is right.
Don't try to do an area bigger that about 1" in diameter or you'll make it too loose. Also keep in mind your intended use, if you want this to be a neck sheath it should fit snug.
One thing you may also notice is that the more you remove the knife it will shave off little pieces of Kydex and develop a looser fit in the real tight areas. You may want to cycle it through the sheath 30-40 times before trying the steps above.
Good luck and please let us know how it goes.
FWIW, I got a Simonich Pikuni and the sheath was great for belt carry but would not hold the knife for inverted neck carry, it was too loose. SO I had to make a separate neck sheath. Fine with me though because I like my sheath to be single purpose so I don't have to mess with switching them around so much.
