Why are Pentax Limited primes so slow?

I've wondered about this for a while. Although I love the small size of many of the Pentax Limited primes, I am puzzled by their relatively small maximum apertures. Many of them max out at f2.8, some even at f4.0. F2.8 is fast for a long zoom lens, but in an expensive prime? I am aware of the built-in stabilization, but that only helps with shake, not subject movement.

The old standard Pentax primes were frequently f1.4, Canon sells a popular 50mm f1.2, plus some very affordable f1.4 and f1.8 alternatives. Leica, Cosina and Voigtlander make f1.0 and even f0.95 lenses for a premium. I am sure the Pentax limited lenses are optically superior at smaller apertures than the cheapo Canon's, but still.

Is it because of physical size? That would puzzle me as I have several old MF lenses that are f1.4 and f1.7, and they are tiny. So, why are the Pentax Limited lenses so slow?