Do you have any auto-sizing turned on, or is your container field and popover size locked? It sounds like auto-sizing/positioning is causing the issue.
On both the popover and layout I have the container format as Reduce image to fit and the alignment set to center graphic horizontally and vertically. For Position, they both have Autosizing with the left and top locked. I experimented with every combination for autosizing. Resizing is grayed out for the container. I even tried copying the container from the layout to the popover so I know they would have the same attributes. No luck.