Why? Because it's in the Constitution! Although, it's not quite that simple…
Section 24 of the Australian Constitution says that the number of members of the House of Representatives “shall be, as nearly as practicable, twice the number of the senators”. Section 24 also explains that each state shall elect members to the House of Representatives based on that state's population. This mathematical relationship between the size of membership of both houses is often referred to as the "nexus".
This part of the Constitution is trying to achieve two things. Firstly, it seeks a balance between the House and the Senate in representing Australians; that is, the proportions will always be about the same. Similarly, it also places limits on the House of Representatives dominating the Senate if a joint sitting of Parliament is held following a double dissolution election. If a joint sitting is held, a proposed law must have majority support in both the House and the Senate if it is to pass.
It is worth noting that as Australia's population has grown since federation, so have the number of elected representatives. Currently there are 76 senators – 12 for each state and two for each territory – and there are 150 members in the House of Representatives.