Yes. A Foot-Lambert is essentially lumens per square foot, for this purpose. So if you spread the available light over a larger area, the brightness is reduced. One way to overcome this is to use a screen with some "gain". A gain of 1.3 over reference white is a typical HT screen, and they go as high as 2.0 or 2.5. The higher gains tend to have hotspotting, directionality problems, etc.