Potato weight less than advetised

Good Morning I am not unhappy with Spar but with the company LUIPERO****RT who supply the potatoes to Spar. Went to purchase a 7kg pocket of potato's and when I picked it up it seemed lighter. I requested the gentleman weighing fruit etc to put on his scale and the weight was 6.2kg, we then went on to weigh a further 10 bags, all under the weight of the 7kg. I eventually settled on 6.9kg still under weight. Surely if the patato's are advised for 7kg, then it should not be less than - this is false advertising.