I wouldn't say it is that clear, there are different levels to this and could be considered risky. I do understand that many people think of the situation a bit differently because they look at it like you, but bigger clubs do tend to play worse against weaker teams since they underestimate their opponent as well. So, it's clear that Real will play differently towards Manchester City compared to how they played against Leipzig.

You can put some second division team against them and find out how bad they could even get, it is very obvious. This is why the best game Real Madrid will play all season long could probably be against Manchester City, not saying it will be, just that there is a possibility of something like that being true.