This is quite normal and also very useful as the main page is still on view. It also means you can have several smaller windows open at the same time without losing site of the main page.
If you still prefer to have new windows open at full size then open the new window but don't use the maximise button to expand, instead move your cursor to the corner edge of the window and when it changes to a double-headed arrow drag the window out to fit the whole of your screen.
From now on it should open in this larger size but you will have lost the ability to reduce to smaller size using the centre button on the toolbar. You can of course always reduce when needed by dragging from the corners again. j.