Why the size of search thread pool in ES2.3.3 is smaller than before(f.e es version 1.3.0 )

My server cpu cores are 24,today,i find the number of the search thread pool is 37 in my es version2.3.3;i want to know why this number is smaller than old version. what is the new default setting perpous?
thx for any answers~

Hi! Take a look into this: