I've long wondered about this myself. Making plastic is pretty energy intensive, but so is making glass. The former mostly comes from new fossil fuels (sometime

I've long wondered about this myself. Making plastic is pretty energy intensive, but so is making glass. The former mostly comes from new fossil fuels (sometimes with some recycled content), while the latter is mostly sand and is very often recycled. Discarded plastic, while hard to recycle, can be burnt for energy, while discarded glass bottles can either be washed & reused, or melted down a recycled. Glass bottles are more than an order of magnitude heavier than plastic bottles. This does seem pretty significant. It would be interesting to see a full lifecycle analysis of both options.