Why is 口が軽い the opposite of 口が堅い ? It seems interesting to me why 軽い／堅い would be chosen for this phrase.
As far as I know, most of the time phrases that are coupled with 軽い usually have the opposite meaning paired with 重い. I know one case where it is paired with 激しい as in 軽く／激しく運動する.
What would be the reason as to why 口が重い might not be the appropriate choice to use for the opposite phrase of 口が軽い?