Didn't think I'd be answering photography questions here... actually, there are probably other people here much better qualified than me to answer, but hey. I'm bored and have nothing betterto do!
OK, the 18-55 lens. I assume you mean this one?
Canon EF-S 18-55mm f3.5-56 IS MkII Lens - 5121B005AA - Jessops - Lenses
It is an EF-S lens which means it requires an APS-C sensor. The 5D has a full frame sensor.
The Sigma lens is also designed for APS-C sensors. If you fit the 18-55 lens to your 5D it would produce the kind of results you are acheiving with the 18-200.