I don't think it's a bug in OBS. From what I recall of the code it just calls for the window to be drawn and handed to the plug-in for all the content in the window and the plug-in decides the size of the window.
I've seen this happen in DAWs as well, so I don't think it's a bug in OBS per se. That said, maybe some more code could be added to force a resize of the window to the viewable plug-in area after the plug-in initializes, but that then breaks other plug-ins (and technically the VST 2.x standard) and could lead to crashes, so the best thing is to just ignore it.
Thinking about it some more, it could also be coming from some type of minimum size window defined somewhere deep in the Qt/interface/C-wrapper of Qt code in OBS, but I wouldn't be able to find that without spending more time then I have to look for it...and I seriously doubt Jim did something like that in the code.