I'm willing to bet that as you chimped (reviewed) images in the camera you deleted some of them.
The content of a digital file influences the image file size.
Put another way, each image file is a different size, in mega bites, not pixels which is image resolution.
When you delete a photo using the camera the image file data is actually still there in memory.
What happens when 'delete' an image file is the memory card FAT (File Allocation Table) is updated indicating the memory locations where the 'deleted' photo data is is again available.
Unfortunately, the size of the next image file over written to that range of memory locations likely does not match the same size as the image file that was deleted .
The result is the new image file gets corrupted because it's size does not match the size of memory space allocated by the FAT for the deleted image.