There is the photosite count of the sensor and there is the pixel resolution of the recording, and they don't always match.
A Canon DSLR can't really record a raw image off of the sensor at 24 fps for any reasonable length of time so the image has to be reduced somehow to a smaller video format for recording.
Ideally any camera would be downsampling from the full sensor but often DSLR's do tricks like pixel binning and line skipping (thus creating artifacts and losing resolution) -- or simply cropping -- or a combination of these things -- to make it easier to get down to a smaller size. It varies by camera so do some homework.
ARRI only offers uncompressed 6.5K raw recording for the Alexa 65 so the files are huge.
The Canon 5D MkIII sensor is 5760 x 3840 -- you could build a camera that recorded 5760 x 3840, either raw, compressed raw (as a Red camera would), or in an RGB compressed format like ProRes, but it would have to have a processor large enough to handle that data and a cooling system that could keep the sensor from overheating while taking pictures 24 times a second or higher. DSLR's generally don't have those features.