I kick off my 150 address range scan on a Monday morning, then kill it Friday evening each week. Over the week, it only ever finds ~70hosts.
I mainly use default scanning options, but I have added ~10 columns made up of mainly WMI queries and a few regkey reads.
I only port scan 80 and 21.
It's not a big concern obviously, I was mainly just curious.
Well that's rather expected I guess. It would need to send at least three ARP packets and one or more ICMP packets to see if a device is there. If you are using version 7.0.7 or above, it would be done in a few threads simultaneously, sending tens of packets per second.
This can be reduced by lowering the maximum number of threads in the settings. The background scan uses 1/10 of the maximum number, so if you set it to 10, then the background scan will be effectively single-threaded, taking longer to complete every round and thus using less data.