I do not use iBooks on my iPhone or iPad. At present, I use Bluefire Reader on my iPhone as it gives the least amount of margins of any good reading app that's not tied to any store/reader brand. Bluefire supports Adobe's DRM so you can use it for your DRMed ePub and even library ePub will work with Blurefire. This is something that won't work with iBooks and iBooks is tied to Apple (DRM). There's a good chance of paying more for your eBooks if you buy from Apple.
Now, as to the Marvin issue. Marvin is only available for the iPad. It's a work in progress. The next version due out is 1.2 and will be ready for use on the iPad (some issues currently make it not really ready (IMHO)). After the iPad version is mostly done, an iPhone version will be written. Marvin does not and will not support DRM. Marvin is for ePub without DRM. I like the way Marvin is going (development).
For the OP's problem of wide margins with iBooks and even Kindle, Bluefire Reader is the solution.