I'm guessing you're using Convex mode for the meshcolliders. Concave would be the mode to use if you want the collision mesh to exactly match the voxels. However, concave meshcolliders come with restrictions, such as not playing nicely with rigidbodies. They work fine for (for example) terrain or anything that doesn't move.
If you need to use convex, then your best bet would be to use a hybrid raycast. You would first cast a ray using normal Unity physics and then when you hit a Volume object, continue stepping along the ray using Picavoxel's Volume.GetVoxelAtWorldPosition() until you either hit a voxel or reach a certain distance to stop checking.
There is an example of this in the Islands demo, take a look at the IslandsClickBoom.cs script.
The volume splitting was just an experiment and isn't part of the main PicaVoxel package because it's probably not performant enough to use in production games. Your mileage may vary, though, so feel free to give it a try: https://1drv.ms/u/s!AhdchalVsPvXnMk861F6BPKYeCbr0w
One thing it doesn't do which it probably should is destroy splits when there are no voxels left active, which would require a count of active voxels after a split takes place.