The form of the adjective is dependent on the gender of the noun it describes. For example, "duży" (masculine), "duża" (feminine), "duże" (neuter), "duzi" (masculine-personal plural) and again "duże" (not masculine-personal plural). Then of course cases come into the picture, but these are the basic forms.
It's hard to explain, but it's mostly because in Russian "duzhiy" is rarely used and means something like big, strong and muscular, when speaking about men. And the word "pyos" is not the main word for a dog, and is used as an insult. So together this looks like a very emotional characteristic of some guy.
Ok, I see. But according to Wictionary "пёс" is a literary word although can have negative connotations. Here I also found something about "дюжий": https://en.wiktionary.org/wiki/doughty
In Polish "pies" can be also a slang word for "policeman". There is also an expression "pies na baby" which is rather pejorative - "womanizer"
I think "пёс" is not so commonly used as "собака", and it has become somewhat fashionable to use as a swearword in the past couple of years because of some videoblogger (but I'm not really sure about this). Thank you for the article! Very interesting cognates, I had no idea it was related to the Danish "dygtig". And thanks for the tip too :)