If I had to guess, it’s probably we (as in the US) consumes more. One takeout meal for my family is probably more plastic in weight than your grocery runs. My plastic foldable table is probably your annual amount. While it seems wasteful, plastic bags are very minimal compared to other items.

Although I agree that Japan uses lots of plastic everywhere, I can see how the sheer size of stuff in the US can easily offset for that. The average American man weighs 50% more than the average Japanese man (200 lbs vs. 135). The average househould trash container in the US is the size of a garbage truck in Japan (being dramatic with this one, but less than you'd think).

Go to Costco in Texas and watch the literal pallets of bottled water the people cart into their trucks. They make up for entire Japanese towns probably.