Australia is clearly the larger of the two: about 7,706,009 km² km² against 2,712,747 km² km², roughly 2.8× at true size. Drag one onto the other on the map above to feel the gap.
Australia sits in Oceania, Kazakhstan in Asia. On the old Mercator map the latitude gap distorted how large each felt beside the other; at true size the answer is simply 2.8×.
The old map muddied it further: Mercator inflates Australia about 1.2× and Kazakhstan about 2.3×, so on a Mercator-style map this pair appears 1.5× apart instead of the true 2.8×.
Kazakhstan fits 2× into Australia.