Some camera models write a smaller JPEG preview file in their raw files. The program shows the dimensions of the preview JPEG that gets displayed.
The codecs have an option to bypass the JPEG preview and perform a full conversion (see the Codecs Configuration Tool), and FastPictureViewer also has its own option to bypass the JPEG preview, see this post
for details, but be aware that images converted from the raw data does not reflect in-camera settings (such as sharpening) and that the color balance might be slightly affected too. For previewing and culling purpose the JPEG previews are usually preferable, and viewing pre-made JPEGs is much quicker than performing full raw conversions.