Hey Erica, I know you posted two years ago but just wanted to let you know that I have a big old head and have had to have two Chiari surgeries. First bony decompression and three years later (1 month ago) duraplasty. From what I understand it's the shape of the foramen magnum, the hole that lets your brain connect to your spinal cord that is misshapen.
I hope you are feeling well,
kizzkell
Erica,
My daughter's Chiari expert explained to us that it is actually a hypo-plastic posterior fossa that causes Chiari, and it is genetic.
A fall may bring on symptoms and further herniate the cerebellar tonsils, but it won't cause Chiari. The "acquired" type is probably a case of misdiagnosed Chiari prior to the injury, which led to the MRI.
So, the answer to your question is YES! It is the base of the skull (posterior fossa) that is the key. If it is too small for the hind brain, then it will cause pressure and sometimes herniation.
You may would find the Chiari Institutes's videos very interesting because they really explain Chiari in depth.
I hope this is insightful,