"It's mental.' Expression for "It's crazy." "Yea, big time." Expression for "I agree completely."

A: There were so many people there today. B: Yeah, it's mental. X > Yeah , it is a big thing A: He is a very ....(important) person. B: Yeah, big time. X .> Yeah , he is big time. How to understand "It's mental" and "Big time"？ " It's (a ) mental( thing )" >something that has to do with how you feel /or something to do with his mind "It's big time" >somthing that is of a high level. Example > The problem she has is all mental , but if she does not control , it can be a problem big time

Thanks very much Doug C, it is very helpful.

for question 1, you used 'were' (past tense) so the response would be: Yeah, it was mental. or Yeah, 'twas mental. it's is a contraction for 'it is' (present tense)