Originally Posted by murg
It really isn't a size issue, it's about how the Kobo renderer deals with svg windows, or about how to define the external size of the svg window.
Not sure if this would help since I haven't played with keeping a near full page image together with a header or caption but have you tried grouping them inside a div? Using group as the class name and removing extraneous odds and sods:
This seems to work for me but the images I was using were smaller and I wanted to keep the image and caption together.