That's actually not how it works.

Firstly, coastlines aren't truely fractals, because matter is made up of fundamental particles, so there is a point where you have to stop zooming because the curve isnt becoming any more intricate. However, atoms are super small, so in the large scale, a coastline might as well be a fractal.

Now, lets talk about the different types of infinity. In the scope we are talking about, there are two types: countable and uncountable. Countable is simple, the easiest example is the size of the set of all integers. This is the infinity that people are normally talking about when they say infinity. There are other examples, such as all the even numbers, and all of the rational numbers. It is called countable infinity not because you can count to it, but because you can count in a pattern, and if you carried that pattern you could get to any number in that set in a finite amount of time.

Uncountable infinity is different. The best example is the amount of all decimals, both rational and irrational between 0 and 1 (or in fact any two numbers that are different in value). It is unaccountably infinite because there is no pattern that you could count the numbers in the set that would allow you to any particular one of those numbers in a finite amount of time. How would you even begin to count the decimals between 1 and 0?

Now, onto coastlines. The length of the coastline fractal we are talking about is uncountably infinite. That is true for both Cuba and Australia. However, both the length of the Cuba fractal and the Australia fractal are the same.

To think about this, think back to the set of decimals between 0 and 1 I talked about. If I changed that definition to say that it was the decimals between 0 and 100, or 0 and 100000, the size of that uncountable infinity would not change. You are zooming out, but infinity is just as infinite.

On top of this, even if these fractals were countably infinite, they would still be the same size. The size of the set of all odd numbers it the same as the size of the set of all integers is the same as the size of the set of all rational numbers. Again, you are changing the scale of what you are looking at, but you can still map them 1 to 1 with every other countably infinite set.

So, what I am trying to say with this super long ramble of a comment is that, in fact, the Cuba fractal and the Australia fractal are the same infinity, neither larger than the other.