In Australia, a normal degree only takes 3 yrs, regardless of how many majors you choose to do in it. If it's a single degree, it's 3 yrs. Unless it's a professional degree (doctors, vets, pharm, etc) or if it's engineering (4 yrs).
Double degrees programs usually last for 4 yrs, and they don't need to relate to each other at all. A lot of times they are in different departments, and really they are treated as two degrees and you can do more than one major in each if you choose to. How do you have time? If you look at a degree in Australia and a degree in America, there's no obligation to learn anything extra other than what your degree is supposed to deliver, no US history, professional practice, etc, if you do physics, you just do physics. That's why degrees are shortened to 3 yrs. Of course you can still pick other things you like to do in your electives. But in a double degrees program, you don't get many electives.