They are similar pushbutton true, but I lived in sheffield for a number of years and Meadowhall would fit comfortably inside Marion IMO, it only has about 250 shops, the food court is tiny and the dept stores are much smaller area-wise than david jones or myers. I'd say marion is a good 40-50% bigger even as it is now. Trafford centre is about similar to meadowhall but impossible to park... and if you do get inside, you soon wish you hadn't!
You could walk end to end in either of them in 5mins whereas I get lost in Marion without my 11yo daughter to come with me
Merry Hill in birmingham is fairly old & grotty, and the malls in london such as brent cross are surprisingly much smaller and older.
The French channel ones are pretty awesome tho, although not a fair comparison with us as they cater for the cross-channel trade for lower GST in France.
A big plus for Adelaide I reckon!