I was browsing Wikipedia to find out how long a Chinese 'chi' was, and it turned out to be 0.3-ish metres. 'Hmmm,' I thought, 'that's roughly the length of a foot!' So, then I found out that a cubit was in the same ballpark as a foot (and uses the same derivation as a chi - the length of a human forearm). Now, from what I understand, the modern foot is actually derived from an Anglo-Saxon measurement that was larger than the Roman foot.
So: Why are all these figures in the same ballpark? I get that a cubit and a chi would be similar, being based off of the same body part (albeit with regional differences), but why is a foot in the same ballpark too?
A cubit isn't really in the same ballpark as a foot. While the definition of cubit varied a bit, it was roughly .5m. That's substantially larger than the chi or the foot. The main question is why the chi is different than the cubit.
An important thing about physical units are that reference points are very important. "Length of forearm" is quite ambiguous. The cubit is roughly the distance from the elbow to the fingertips, which comes out to roughly half a meter (and there were historically various standards based on that). Presumably the chi uses reference points that happen to be approximately one foot apart on an average person.