Not only is the body size difference considerable, but lens size is also a huge factor in the weight and bulk of the total package.
Take a look at this shot, with an Oly 9-18 sandwiched between a 24-105 and a Sigma 17-50. I'm amazed at how tiny these things are.
BTW, it's a great little lens, one of the only m43 zooms worth owning. Plus, it's a joy to carry around.....
9-18mm is on my future purchase list. When I saw it in person it was almost impossibly small--noticeably smaller than even the collapsible kit zoom. But I'm not much of an UWA shooter so I consider the price about $200 too high. If it ever hits $500 or less I'll probably jump on it.