ReportViewer will resize correctly when the window is resized by manually dragging the frame, but when 'Maximize' or 'Restore Down' buttons are used, the resize will not occur. 

If continues scrolling is turned on, the next page to load in will be in a different size because it will be in the correct size, while the first page remains in the previous size.

A work-around is to add this codesnippet to the HTML, which will trigger the resize:

function resizeViewer() { $("#reportViewer1").trigger('resize'); }
window.addEventListener("resize", resizeViewer);