Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 609)

3.
Select the size of images to transfer.
Select [ JPEG size to trans. ], then select [ Larger JPEG ] or
[ SmallerJPEG ].
To transfer smaller JPEGs when the camera is set to record larger
JPEGs to one CFexpress card and smaller JPEGs to the other, set
[
JPEG size to trans. ] to [ SmallerJPEG ].
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