It's a variety of factors, some geographical and others economic, but the most significant is that Western Australia was settled very late, and did not gain the demographic traction of the East. Western Australia is smaller and dryer than Eastern Australia, but not to the point that demographics might suggest. It is mineral rich and a massive producer of agricultural products. At Federation it accounted for 5% of the population; now it is more than 10%. The Pilbara and Kimberly in particular could support two orders of magnitude more people if properly and sustainably developed.
To an even greater extent than the rest of Australia WA is simply underpopulated, and did not receive anything like the immigration that it could support. It will become much more heavily populated over time though, and while Perth will probably never reach the size of Brisbane, Sydney or Melbourne (all of which are projected to be somewhere in the vicinity of 10 million in the latter half of the century), but its proportion of the total will continue to grow. The people simply need to come into existence.
To an even greater extent than the rest of Australia WA is simply underpopulated, and did not receive anything like the immigration that it could support. It will become much more heavily populated over time though, and while Perth will probably never reach the size of Brisbane, Sydney or Melbourne (all of which are projected to be somewhere in the vicinity of 10 million in the latter half of the century), but its proportion of the total will continue to grow. The people simply need to come into existence.