Most things don't have significant economies of scale beyond a fairly small country (maybe the size of England). The 3 biggest polluters are electricity&heat, agriculture and transport.

- Electricity and heat benefit from a bigger electricity network (e.g. to get more reliable solar and wind), but electrical grids don't necessarily follow borders. The US has about 3 grids, continental Europe has a grid spanning 24 countries. (in fact, there's almost no grids that span exactly one country [1])

- Agriculture doesn't hugely benefit from economies of scale beyond the regional level

- Transport benefits mostly from rail networks, which aren't really bound to borders

You can make a case that population density helps (most obvious in the case of transport, but it is really helpful for lots of things), and being an island makes electricity harder. But in the case of Australia, it has to be noted that the negative effect on electricity from being an island is somewhat lessened by their vast area, and the negative effect of low population density on transport emissions is somewhat lessened by most Australians actually living in pretty high density.

https://en.wikipedia.org/wiki/File:Wide_area_synchronous_gri...