Graham Poll admitted it years ago. And i think he would know more about it than any of us..
Poll.... "Let me guarantee you this: none of the penalties given to Wigan Athletic, Stoke City or West Bromwich would have been awarded against Manchester United and yet they were against Norwich City. So are match officials really biased, do they deliberately favour the big teams?
The select group of referees which I served with did a lot of work with a sports psychologist, the excellent Craig Mahoney. He explained the difference behind conscious decision making and sub-conscious and that you can only really control conscious thought.
I don’t believe that any referee consciously goes out to give soft penalties against small teams but I know that they only give stone wall penalties against the biggest ones.
One of the reasons for this is that they know the fallout, from the managers and the media, if they give a soft penalty against one of the big teams and the sub-conscious mind kicks in to afford the referee a level of protection, quite naturally."