The concept of "more is more" is easy to understand; do more to get more. Seems obvious. So trying to understand "more is less" is a bit of mind-bend...how can you do more and get less output? Well, it's the point of diminishing returns, where you've reached a level of optimal balance between stress and rest...but then push a little further / harder in search of more, but actually get less because you're not coping with or adapting to the workload. Then performance, mood, energy and even health can suffer...more is most certainly less.
I have mentioned before that when things are going well, be conservative. This means that when things are in a nice balance, and all of the stresses in life - work, family, exercise, etc - are balanced out with sufficient rest and recovery, status quo is maintained. Within the scope of those elements of "life" you still have plenty of opportunity to advance and progress - they are like pillars holding up a roof; they share the load, even if the roof shifts and changes a little.
But when you try to put more / too much emphasis into one element...for too long...one of those pillars begins to take on too much weight to support and in the end it is likely to fail, and the whole roof will fall down. You are far better to find the point of optimal balance between work and rest, across all aspects of life - and especially your physical training - so that you can keep on progressing in a way that is compatible with your *own* limits of tolerance. The right amount is the right amount - with a little wiggle room for temporary changes - but more is often less.