If you can obtain an A.P.A. "Standard of Perfection" book, there are great illustrations of the body types that you can compare.
Also on line you can find the standards and pictures of both breeds to see the differences.
Orpingtons are considered a HEAVY breed, large with no sweeping back but almost flat with a slight upward angle to the tail. The plumage is very loose and fluffy compared to the Australorps with sweeping "U" shaped back and nice tight plumage.
Orps also have a small to medium sized comb compared to a large tall single comb of the Australorp.
Go visit a poultry show and you will definately see a difference in the body sizes and widths.
Both breeds are great to own, funny, freindly, talkitive, dual purpose and beautiful !
Thanks. I have Buff Orps and next month I'm getting 3 Lavender Orps. I have 5 baby Australorps that are 5 weeks old, they are just hatchery chicks from the feed store, but I like their friendliness already!