Miedema netted three times in each half and also provided four assists as the reigning champion moved top of the standings.
Lisa Evans netted twice, while Jordan Nobbs, Leah Williamson and substitute Emma Mitchell were also on target for Joe Montemurro’s side.
The result surpassed Liverpool’s 9-0 win over Doncaster Belles in 2013.
Bristol City, which is now eight games without victory this season, pulled one back five minutes from the end when Yana Daniels scored on the rebound after her penalty was saved by goalkeeper Manuela Zinsberger.
The record win saw Arsenal