The EOS r5c is much bulkier in the back. This is to accommodate an active cooling system so it can shoot for long stretches without overheating.
The r5c also supports 8K HDR recording in HLG and PQ format as well as 4k/120fps recording in camera’s three-way power switch lets you boot it up into photo or video mode. The full-frame sensor is 45 megapixels.