Originally posted by normhead Sigma engineers are famous for adding weight to achieve quality. Understanding how to achieve quality while keeping weight down is not even in their mind set
It's not so much
adding weight as not taking active steps to keep weight (and size) down.
Pentax engineers have shown themselves ready to sacrifice f-stops in order to prevent explosive growth of size and weight (e.g. DA Limited lenses), and to sacrifice ultimate wall-to-wall sharpness and allow odd focal lengths to emerge* in search of a particular image quality, so it's really just a matter of design philosophy and where you're prepared to compromise.
*
I'm fairly convinced that the FA31, FA43 and FA77 are a result of a design philosophy that said "We want a wide, a normal and a portrait-length lens with the following optical characteristics", and the specific focal lengths weren't even known or cared about until they were computed at the end of the design process.