My first though would be that you could invert the scale so that the knob remains the same size. For example, if the entire symbol was scaled 50%, then you'd perform an opposite scale of 200% on the knob.
I was hoping that this would work with the Symbol Override Extension, but it appears that persistent symbols are not handled with it (paging @MarkHunte).
You could do this manually in a little bit of code instead. If you give the "Ellipse2" in "Symbol D" a Unique Element ID of "ellipseD" via the Identity Inspector, you could use this code On Scene Load to change the scale:
hypeDocument.setElementProperty("ellipseD", 'scaleX', 2.0);
hypeDocument.setElementProperty("ellipseD", 'scaleY', 2.0);
You'd of course need to add this to all scenes.
If you wanted to get fancy, you could probably base the scale on a parent element, and even use a global HypeSymbolLoad event to always run it. If your needs are more advanced and you are familiar with code, it might even just be easier to get the value of the timeline and save it as a global and transfer it to other elements. But these might be overkill 
.