Keyframe shapes: size fixed, speed on fader, how to?

[Il Tassp]

Hi all,
as per subject (2007 Pearl Expert with Titan 9):
- I create a keyframe shape,
- record it on a playback fader,
- go to Playback options for that fader,
- set Size: fixed and Speed: on fader.

Keyframe Speed is now linked to the fader position, from 0% to 100%, OK.
But no matter how hard I try, also Keyframe Size remains linked to the fader position, while I would like to have it always running @100%.

What am I doing wrong?
Thanks.