I can see all info in full screen mode in portrait but not landscape. Why?

On LG V30 can use full screen when phone in portrait mode, but most of screen is black when turn phone with same image to landscape mode. Why?

In general, if the shape of your picture does not match the shape of the screen, you will get black margins since the default view mode for most picture viewing apps is "fit with no cropping".

Which picture viewing app are you using?
If you need a better one, I highly recommend "piktures" by Diune.

Some screen shots might be helpful so that we can see what you are seeing.

I seem to have fixed the problem by scaling down Display size from totally + to halfway between + and -. Now when I am searching web and I rotate a view to landscape, instead of getting a mostly black screen, I can see the view in landscape. Thanks very much for responding.