hello,I have a problem about lucene,please help me to explain ,thank you

[王建军]

    There is a Class org.apache.lucene.codecs.blocktree.BlockTreeTermsWriter 
which have two parameters,one is DEFAULT_MIN_BLOCK_SIZE,the other is 
DEFAULT_MAX_BLOCK_SIZE;their default values is 25 and 48；when I 
make their values to bigger,for example,200 and 398；And then to make index，the 
result is that the use of memory is become less，what's more ，there is a good 
performance。
    Can you tell me why；what's more，if I change that ,whether or not will make 
other problem。
    
    Thank you very much。