HTTP transfer -> fast. SMB transfer -> slow

Hi all,

I've got two Win2003 Servers at different locations. If I transfer a
file between them using HTTP I get 10Mb/s (which is the bottleneck
speed of the sites' Internet connections). I can do the transfer in
both directions with the same result.

Based upon this, I deem the network "working properly". So far, so
good.

However, if I try copying the same file using Windows Explorer or
robocopy, I get about 5Mb/s. This is not fully utilising the bandwidth
between the two sites.

An Ethereal capture reveals no packetloss or retransmits in this latter
case. It just doesn't go as fast as an HTTP transfer.

Is there anything I can do to speed this up?

many thanks,
alec