To whittle down a group or thing means to gradually make it smaller.
He had whittled eight interviewees down to two. [VERB noun PARTICLE + to]
By September, they will have whittled the list down to a winner. [V n Pto/from num/n]
The president has agreed to whittle down his proposal. [VERB PARTICLE noun]
[Also
V n
P,
V n
P +
from]