Every breed is different, size plays no role in maturity of breed necessarily. On my yard, I have Shamo which are considered to be a slow maturing breed (takes them about 2 years to be truly mature), the hens weigh about anywhere from nearly 7 pounds to nearly 8 pounds. My cock that I have left is 9 pounds 6 oz's and nearly 3 feet tall. At the same time, I have Buff Leghorns which are very small in comparison to them, but Leghorns are only supposed to get up to 6 pounds for a cock and being hatchery stock they may not be that large. I want to say the little cockerel is about 4 and a half now,
I haven't weighed him.
I have Kraienköppe that look like they are almost bantams in comparison to my Shamo, yet all of them are still breeding. They actually are nearly a bantam, some of them are only around 3 pounds. American game come in various sizes as well, I've had birds as small as 4 pounds 8oz all the way up to over 6 pounds. The Australorps, if I am correct, are large birds usually weighing in the 8 pound range I believe? I know the one my aunt had probably would have fit in well there; he wasn't a tall bird per-se but he was a wide one.
Even if it is smaller, the birds will still fly well or better. I had Old English Bantams for a long time and I saw those little birds do stuff I've never seen any chicken do, they were one step away from becoming a song bird they flew so well
. I imagine the Australorp are similar to my Shamo, the wings won't be big enough to support the large bird so they won't be able to fly well. Same problem with the Rhode Island Red's I have. I've never seen either breed fly higher than about 5 foot off the ground whereas my games, Leghorns, Kraienköppe, etc- can fly straight from the ground up on to a branch that is 8-10 feet in the air without it even bothering them.
God bless,
Daniel.