http://skyscraperpage.com/diagrams/?searchID=52603538
To me that is a large difference, to you it may not be. It's all relative. I also think TwoPru is a dwarf compared to Aon Center, but you may not.
http://skyscraperpage.com/diagrams/?searchID=52603568
And I think Aon is a dwarf compared to the Willis/Sears.
http://skyscraperpage.com/diagrams/?searchID=52603572
In terms of RSF the Twin Towers were each a good 50% larger than Aon Center, which is very substantial.