Reading that document on Macromedia's site gives me light.
http://www.macromedia.com/support/flash/ts/documents/beziers.htm
It says, swf format only supports quadratic beziers, what less accurate than cubic beziers, but easy to render fast. I think, in swf file's beziers are quadratic interpolation of better looking quadratic beziers. Threshold value drives this accuracy (more control nodes, bigger files)
ming_setcubicthreshold
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User Contributed Notes
cece at php dot net
29-Aug-2002 01:18
29-Aug-2002 01:18
rob at theblip dot please dot god dot no dot spam dot com
10-Apr-2002 03:35
10-Apr-2002 03:35
From the source code: sets the threshold error for drawing cubic beziers. Lower is more accurate, hence larger file size.