Most likely someone just picked a number at random. For things like this, there
is a very wide range of values that are probably fine. The main limiting factor
at the high end, is some older file systems still don't support files >4GB, so
you'd want to pick a value below that; Monero all block data in a single file,
and that occasionally leads to issues when, eg, copying that file from one
place to another. At the low end, having tens of thousands of files in one
directory tends to cause problems.
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