Select a clip in the timeline, change spatial conform to none, make it a compound clip, then change transform to something less than 100% (also notice where the transform box outline is when you enable the transform effect in the viewer), apply any effect (as far as I can tell; but I used Gaussian Blur as one example). Notice how applying an effect crops the clip to the transform box outline in the viewer which is not 100% of the clip size when none is applied to the nested clip (inside the compound clip).
Effect disabled:

Effect enabled:

I can stick with fit rather than none for compound clips, but it would be nice to be able to reference image size relative to the source rather than the timeline in certain circumstances.