Getting/Forcing Greater than 4KB Buffer Allocations

I'm investigating a problem with my bce driver which occurs when I ask
for a jumbo
mbuf cluster (through m_cljget()).  When I map the memory for DMA I
normally 
get 3 memory segments (4KB + 4KB + 1KB) on my system, but on another
user's 
system he's seeing 2 memory segments (8KB + 1KB).  Is there a
configuration
option that allows this or some other tuning variable involved?  The
system is a 
Xeon dual-core processor and has 8GB of RAM, running an AMD64 version of
the kernel.
 
Dave