Complex sentence: Round glyphs will break (overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing; a rounded shape will appear visually smaller than flat topped (or bottomed) shapes of equal height, due to an optical illusion.
Split into simpler sentences: Round glyphs will break the mean line slightly in many typefaces, since this is aesthetically more pleasing. As well a rounded shape will appear visually smaller than flat topped (or bottomed) shapes of equal height, due to an optical illusion.