Bind 8 vs. Bind 9

[Doug Hardie]

I have a large mail server with a couple of zones defined where the sum 
of the zone definition files is 153 MB.  When I use Bind 8 the VSIZE 
for bind jumps to 250 MB.  Thats with nothing going on using bind.  
When I switch to Bind 9 and load the same files the VSIZE jumps to 353 
MB.  I was hoping to use the max-cache-size feature in bind 9 but the 
extra size of it makes it impractical.  Why is it that much larger?