[Brightcolours]

That is the difference between an old fashioned 1970's kind of design (the optical formula of the EF lens is not very advanced) and a modern design.

Here is the difference between an old fashioned "1970's kind of design" f1.4 and a modern one.

  Screen Shot 2018-12-03 at 12.01.17.jpg (Size: 185.62 KB / Downloads: 14)

Note how the short flange distance allows the faster lens to be about the same size, while both have about the same number of lens elements.