How are you judging the “size” of the images: by their pixel dimensions or how large they appear when you place them in another Adobe product like Photoshop or InDesign?
Translating the two sets of dimensional information into pixels for clarity:
"result": 13x19 cm = 5x7.5 inches @ 300 ppi = 1535x2244 pixels, approximately.
"normal": 40x60 cm = 15.75x23.6 inches @ 240 ppi = 3780x5670 pixels, approximately.
The 5D.3 native size is 3840x5760 pixels, so slightly larger than what is described as "normal" but perhaps within rounding error of the cm units.
As far as what might be happening, the most obvious two things are that you're cropping the image smaller, or that you're resizing it, despite what you say about not downsizing.
To help understand if there is any resizing occurring, what are your Image Sizing settings in the Lightroom Export panel? This is where the image size is determined beyond what the original's pixel dimensions.
The only way to be sure you’re not downsizing is to have Resize to Fit unchecked so no resizing is occurring and native pixels are being produced.