The first time I had this issue was on the Nightstone map on SKT module from WotC. It is a biiiig map with a lot of squares/grid. The second time is on a non-gridded overland view of a town (Triboar from the same module). So the image size isn't the problem causer.
I'm not sure what you mean by resizing the layers? I right click the image and then select Adjust Horitontally to make it fit my screen but that shouldn't be a problem.
I have moved the grid around in the Nightstone map but that can't be the cause either because the other map doesn't have a grid but still has the same prob.