Metal typefaces commonly vary in proportion from one size to another, especially in small sizes.
One reason is that fine lines tend to disappear in small sizes and look too coarse in large sizes. Another reason is maintaining legibility in very small sizes and a greater sense of relationship between sizes.
Two leading designers had opposing views on this principle of visual reproportioning, or optical scaling. While Morris Benton's designs for ATF are at the one end of the spectrum, Frederic Goudy’s designs are at the other end, showing little or no modification among differing sizes. (McGrew)
Linn Boyd Benton adapted the use of the pantograph (a machine used for producing wooden type since the 17th century) to metal type. The punch-cutting or pantographic engraving machine he invented enormously simplified and also made the cutting of metal punches cheaper, but it came with a drawback, which was the mechanization of the design of type. Benton believed that a specific type design can only be successful at a particular size, and cannot be successfully applied to all other sizes if it is not adapted to the size in which it is to be viewed. (According to Updike, ideally, a new model design should be made for every two sizes of type.) So Linn Boyd Benton made sure his machine was capable not only of scaling a single font design pattern to a variety of sizes, but also of condensing, extending, and slanting the design (mathematically, these are cases of affine transformation, which is the fundamental geometric operation of most systems of digital typography today, including PostScript). Morris worked on many of these machines with his father at ATF, during which these machines were refined to an impressive level of precision.
In the image below, the first line is set in 8-point and enlarged 50%, the second line is 12-point actual size and the third line is 24-point reduced by 50%. If you click on the image you can see Century Bold Italic among other typefaces for comparison.