There is no significance.

Other vendors should not care about how large your 1st segment is. A small size could give you an incorrectly calculate host speed, since the transfer might go very fast. This would influence the size of the 2nd segment requests. Also there is more overhead compared to the transfered data.

A larger 1st segment could block a bigger part of the file to a slow host. But generally larger segments are preferred when you host speed is not an issue and you don't care about a few slow hosts blocking the last segments.
Larger segments cause less transfer overhead since more data is transfered with each request.

The follow up requests to the same host will size the segment according to the target transfer time and the calculated speed of the host.

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