Odd Memory problem: SA NOT using available memory

I'm out of my element on this, so please forgive me if I ask something silly.

We have two SpamAssassin (v3.002003) servers on Red Hat, each with 4gb RAM.  ServerA ROCKS while ServerB SUCKS.

So I've been comparing the two systems hoping to find some configuration differences between them.

When I do a vmstat, I see a couple of things that are glaringly different:
ServerA
procs                      memory      swap          io     system         cpu
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 0  0      0 354012 154360 3395248    0    0     1     6    6     1  1  2  4  8

ServerB
procs                      memory      swap          io     system         cpu
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 1  0      0 3831400  73308 148168    0    0     3   184  108   325 18  3 68 11

ServerA (the good one) is using nearly all of the 4gb of RAM available to it.
But ServerB is only using a fraction of available RAM and the # of context switches is tons higher, too.

I hope this isn't a silly question, but how significant is this, and where do I go from here?

Thanks for any help,
Joe