I interpret it as emphasizing the colloquialism. Also it seems to wave towards the perceived significance of the statement. "I heard Marvel is releasing three

I interpret it as emphasizing the colloquialism. Also it seems to wave towards the perceived significance of the statement. "I heard Marvel is releasing three movies in 2021" "You should take that with a grain of salt" "I heard Marvel and DC are doing a Justice League / X-Men crossover in 2021" "You should take that with a big grain of salt"