You do not increase the size of mount point, you increase the slice/ volume which is mount on the mount point. To do that you need to find what kind of device is mounted on the given mount point. If its volume then you can increase the size with growfs, if its slice then you have to backup the data increase the size of the slice, label it , run newfs on the slice and then mount on the mount point after that you can restore the backed data into the new slice.