Seems that after creation, the size is never again updated / queried.
When the font is auto-scaled, this now functions correctly.
When the imageset is auto-scaled but the font is not auto-scaled, the issue persists. This is because the rendered size of the images change, but - as used to be the case for font - there is no notification of that change.
Images could be made to fire events like Fonts now do, the only problem with that is that it would cause an explosion of events - literally thousands, each triggering a hander on every window. It's not something I like contemplating :)