I was making the scene Aspect ratio 1×1 & resolution 2000×2000. I found thiss issue, Cam view is filling all resolution in the view and cant control it.
BFA border fill all window and crop the view in Blender it is opposite border stays the same only cant be zoomed in/out.
What you search can be found in the Properties Sidebar. Open the View tab. Expand the lock subtab. And here untick Lock Camera to View. And when done, tick Lock Camera to view again so that you can go on with the usual navigation in camera view.
There is another way too, that fits the camera view in width into the current view. It’s in the View menu. The menu point View Camera Center.