Plugin initial window size won't be set

Hi, there.

I’m developing my first Juce-based plugin. The build targets are Mac-AU and Win-VST3. Everything is working fine except for one thing:
The initial window size is not what I have intended on Win-VST.

The window size of the plugin is basically fixed, but only when the user opened/closed an ‘advanced’ panel the resizing happens.
The initial size is specified in the PluginEditor constructor.

This works fine (meaning the window size is set as I specified) with Mac-AU and Standalone executable for both OS. But as far as I’ve tested with Reaper(Win) and VSTHost(Win) the window size is smaller than specified and the contents are just trimmed.
The intended window size is 640x540 and will be 100px bigger when the ‘advanced’ panel is open.

What causes this difference? Has anyone here experienced the same thing?
Any hint is appreciated.