I contacted DMG Audio and they were kind enough to analyze this problem in more detail and explain it to me.
He ran a debugger and could not see any call to IPlugView::setFrame(). which rings alarm bells for any plug-in that can resize.
Essentially Media Center doesn't appear to provide a way for a VST3 plugin to inform it of UI size changes.
As I showed in my screenshots, this doesn't only affect DMG Audio plugins, but also many other plugins from different companies. As far as I know all these plugins can resize.