If you put too little of the batter or dough in, the loaf will cook too fast or rise too much in the middle and if you put too much it may be undercooked in the center and get overcooked on the outside just to get the center done. Just make the size similar to what they call for.
And I have found that glass pans make a crustier and thicker brown area than aluminum.
Ditto on Lisa's post - if you get the quantity about right for the shape and depth, you're bakin'!