I'm using the Outpost Pro firewall and that doesn't use 35MB of RAM (18,392KB of VM Memory, more accurate reading of memory in use) on my system and I would have thought the Outpost free firewall would use less, given it has less functionality.
My concern over Kerio a) it is no longer supported and b) is rules based which to a point you have to create the rules if you really want to tighten down security.