It looks like you’ve set your plugin to scale to 75%. See if it’s more accurate at 100%?
The VST UI interface isn’t that great at communicating/agreeing on size of windows between host and plugin, when the plugin tries to change its size.
Some vendors (cough native instruments cough) “solve” this by simply not resizing/scaling their plugins, which is even worse…
Note that some vendors have managed to make this work okay. XO window scaling works great, for example! So, Plugin Alliance is likely doing something a little different, and the window sizes end up not quite matching up.
Note that Cubase will always have SOME window framing around the plugin, because it implements host-level bypass, patch save/restore/compare, automation read/write, and so on.