Read Tamron's brief explanation of how their use of special glasses helps them to decrease the size of their lenses.
Tamron technology
in a nutshell, exotic glass allows much greater bending of light per lens element, permitting the design of a smaller diameter lens.
Therefore, a smaller lens may cost more than a larger lens and have the same light gathering power. Better glass and coatings should also allow better transmission of light (t-stops) which is separate from f-stops and usually overlooked when we buy lenses.
The same thing can be observed in, say, binoculars or scopes, with smaller, expensive optics outperforming cheaper optics with a larger diameter objective. Unfortunately, only aperture (which is partially mechanical) is ever listed when you buy a camera lens, but transmission of light (an optical quality) should be specified too.
On an aside, DA limiteds are all 49mm - the advantage here is only having to buy one circular polarizer to fit them all.