Sweden is a huge country with very low population density and one of its neighbors is Russia, a formidable military power. This means it is very dependent on the US for its security, and more likely to be vulnerable to explicit or implicit pressure from the US than an island nation of 60 million people that is also a nuclear power.
I'm convinced this is also the case with Australia, which is why they're so unwilling to stand up for Assange.
They don't actually share a border with Russia, were never part of Russia or the USSR and are part of the EU. There's zero chance of the Russians invading.
You do realise the British nukes are Polaris missiles bought from and serviced by the United States, correct? Put blunt, the UK wouldn't be a nuclear power right now if the US didn't sell them ICBMs.
I believe the "special relationship" between the US and the UK would still stand if the US asked for an Australian to be extradited from the UK to the US. The British just extradited O'Dwyer to the US for civil offences. Assange would most likely be charged under the US Espionage Act, which is a criminal offence.
I'm convinced this is also the case with Australia, which is why they're so unwilling to stand up for Assange.