snmpd reduce memory usage

Hey,
 
While trying to reduce snmpd memory usage, 
and going over old mailing-list items.
 
I think I understood from one of them thatm initializing the snmp
librarie, invokes parsing of MIB text files, and loading them to memory.
 
I set MIBS environment variable to empty string (instead of ALL), 
and my master-agent memory usage (RSS column in ps -A v), reduced from
~5MB to 2.6MB.
 
I find it a bit confusing:
 
1) what exactly doess the environment variable MIBS affect?
     as I understand it it affect only the MIBs which the snmp libraries
will be initialized for, but it does not decide which MIBs the snmpd
loads. right?
    1a) why in the firstplace are MIB text files parsed and loaded to
the memory of the snmpd? what use does it have for them?
 
2) what exactly do these configure tokes affect: --with-mib-modules,
--with-out-mib-modules ?
    
 
thanks,
Erez