On macOS the built-in nettop will show you a list of processes that are using the network.
nettop -dPDelta mode, activated by pressing d or starting up with the -d parameter will show you the instantaneous use, in other words you can spot exactly which processes are using bandwidth right at this moment. The -P is to start up in collapsed mode (c while using the utility), so you see total use per process, instead of per connection.
You can also show only the columns you are interested in:
nettop -dPJ bytes_in,bytes_out