For me the size and shape of a 5D body is almost perfect with a long lens (70-200 or larger). I love my M5, but I've tried it with my EF-70-200 and 300 f/2.8 and don't like the way it handles. I have larger than average hands and often the controls on the small M5 body are difficult to work since they are so close together.9VIII said:The idea that small bodies don't work on large lenses is still just a common misconception.
For my first year with the 400f5.6 it was paired with an 1100D and as soon as I tried the 5D2 it felt totally unnatural.
The big bodies are just as detrimental to portability no matter what lens you have (barring something like the 1200mm f5.6).
When you're carrying and moving a big lens you're mostly accounting for size on one axis, if your camera body is close to the height and width of the lens then you can almost store the whole thing in a tube.
Whether sitting down or hiking, the smaller body allows the entire system to lay flat with your body, making it much more comfortable to hold and carry, not to mention doubling the weight of your camera body is always going to be a bad idea when you're hiking. However you can save weight, do it, the lens is necessary to get the shots but a heavy Full Frame body is not.
The latest Rebel Autofocus system (800D/T7i) is fantastic anyway and even supports f8 shooting.
Ideally Canon would take the SL2 and give it high speed internals, better AF, and maybe a Carbon Fiber body to make the Ultimate Wildlife Camera.