Measuring methods are also very different. I've had this argument before here.
Looking at the population of greater Melbourne has you looking at suburbs like Werribee, Frankston, Boronia, etc., which everyone would consider as a part of Melbourne (suburbs, outer, but very much a part of the core).
On the flip side, the "Seattle Metropolitan Area" consists of:
Mt Rainier. Bainbridge Island. Glacier Peak in Mt Baker Snoqualmie National Forest. Mt Vernon. Olympia. North Bend.
No Western Washingtonian is calling any of those locations "a suburb of Seattle".
Looking at the population of greater Melbourne has you looking at suburbs like Werribee, Frankston, Boronia, etc., which everyone would consider as a part of Melbourne (suburbs, outer, but very much a part of the core).
On the flip side, the "Seattle Metropolitan Area" consists of:
Mt Rainier. Bainbridge Island. Glacier Peak in Mt Baker Snoqualmie National Forest. Mt Vernon. Olympia. North Bend.
No Western Washingtonian is calling any of those locations "a suburb of Seattle".