If the material is deforming, choose a larger drill bit and determine the diameter by trial and error.
If it’s not a material issue, there are two possibilities. First, there’s an issue with the drill bit’s grinding angle, resulting in a microscopic C-shaped hole.
For deep hole drilling, consider insufficient drill bit rigidity. Replace the drill bit or modify the process.
Unless the material being drilled has very large elastic deformation, the hole diameter will not shrink.
