Hi....Yes herniations can grow larger and it is also possible that they remain the same size for long periods of time...
Related conditions can affect if or when the herniation grows.....
ie- tethered cord
Regardless of the herniation size, tonsils displaced can cause symptoms.....
There are far too many Drs that do not understand what Chiari is and only look to how large the herniation is ...Chiari is NOT the herniation but a malformation of the skull......but since while in MedSchool not much time is spent on Chiari Drs often over look it or dismiss it completely as they just do not know.
May I ask what symptoms you are having? Are you able to contact a true specialist to gain another opinion?