According to the manual, “… the box* keyword arguments may be supplied with multiple entries to denote the unchecked and checked state”.
Now, for “boxImage” this indeed seems to be true. However, I have yet to find another box* keyword for which this is likewise so.
Are there any other box* keywords that can be specified per-state, and if so, which are they…?
(This as a result of wanting to have a different “boxImageScale” per state, and finding that it didn’t work.)
I did some reading of the code and it does indeed appear that only “boxImage” supports being supplied multiple entries.
For “boxBorder” and “boxPlacement” this is because the method “setFrameSize” in the DirectCheckButton class does not handle that case.
“boxImageScale” and “boxImageColor” is mapped directly to “image_scale” and “image_color” for the indicator (or the box). Thus being set by the “configure” method of DirectGuiBase which simply sets the same value for all components in a component-group.
For “boxRelief”, since the “relief” keyword does not handle multiple entries for different states, neither does “boxRelief”.
You could probably just get the different “image” components from the indicator and set their scale directly to deal with your issue. But it would probably be good if the manual (or the code) was updated so that they matched each other a bit better.
Ahh, thank you for that detective work, and for the thorough answer! It’s appreciated! 
Yeah, it does seem a minor pity to me that the other components don’t support state. But I suppose that such is seldom-used.
(For my case I ended up just changing my direction a little such that I didn’t need different states, and am sufficiently happy.)