i have a gigantic roll of single ply that one of my g cat hoodlums stole. i was going to see how long it would last me but i ended up having to share it, so the experiment is over. this thing is like a 1 foot wide of paper.
Count the sheets on the double roll and see if there really is 300. If so their claim is true that they have twice as many sheets but are skimping on the thickness, which explains the weight difference. While you are at it see if they actually have 150 sheets in the single rolls. Will check back to see your results.