Your video is anamorphic. If you are working in HD it is most likely recorded squeezed at 1440×1080 pixels and that is unsqueeezed on playback to 1920×1080 to display properly. QT Pro is simply exporting anamorphic stills from your anamorphic video at 1440×1080, which is their native pixel dimension.
You can either bring the stills into an app like photoshop and scale them to 1920×1080 to make them display properly, or export your entire timeline to a full raster (non-anamorphic) format at 1920×1080 and then export your stills form that.
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