You have max-width: 100%, but 100% of what? Of the parent width, right? But the parent is an inline-block (with class=”sponsor”) whose width is not set, so its width depends on the children, and in particular on the preferred width of the children.
The layout of this styling is undefined in the CSS specification. In particular, the intrinsic width of the kids in this case depends on the width of the parent which in turn depends on the intrinsic width of the kids. See http://www.w3.org/TR/CSS21/visudet.html#shrink-to-fit-float for the relevant spec text and note all the “does not define” bits.
I recommend giving your <div class="sponsor"> a width. That should deal with the problem, I would think.