I'm very big with the under-5 set.
A: Oh, yeah, mrs. Huber told me about you. Said you illustrate children's books.

B: Yeah, I'm very big with the under-5 set.

question:
1. be very big with
2. the under-5 set

What do they mean ? Please help.

Question

I'm very big with the under-5 set.
A: Oh, yeah, mrs. Huber told me about you. Said you illustrate children's books.

B: Yeah, I'm very big with the under-5 set.

question:
1. be very big with  
2. the under-5 set

What do they mean ? Please help.

Answer

'I am very big with' 
I am very successful with 
'to be big with something means to be succesful ,powerful in achieving it.

'under 5 set'
group of books for children under the age of 5.

So , he is very successful and doing a great job illustrating children's books under the age of 5.

Answer

: Yeah, I'm very big with the under-5 set. = I am very popular with children under the age of 5.