The problem with shopping bags isn't the mass-equivalence, it's the form. Apparently shopping bags are close to the perfect design for killing the maximum numbers of wildlife and for spreading great distances on wind and water. It's why that specific form is fixated on by researchers.
If that's the real issue, then the problem isn't the bags, the problem is people littering. Though, good luck on preventing that.
Extremism, tribalism and simplistic catchphrases
What's scary is those same traits seem to be at work in the US as well.