I agree with this.
My first analogy was going to be chocolate but then again there’s a bar sitting in front of me, so let’s go with language.
I began learning German years ago; eight, I believe. Possibly even nine. Anyway, I found it very easy at first – you know, the beginner’s stage where you learn greetings, discuss the weather and talk about family. I focused on it, determined to master it. And so I kept up the revising, practicing in class with the hesitation that comes with using your second language.
Then I got to university and did a couple of intermediate language courses – much of it was just revision, to me, and I was grateful for the refresher. This is about the time I began trying to read whole books in the language, and it made me frustrated and sad that after six years I still couldn’t get what the book was saying – or I could if I read the English version as well, in which case it was like snapping together jigsaw pieces.
So with too much practice I made myself disappointed and upset that I didn’t have a better grasp of the language. And I sort of lost a bit of enthusiasm. Rather than delight at mastering a tricky word I looked at words as things to be Conquered, to be Memorized and Locked Away in the safe of my brain.
With this in mind, I agree that too much is about as bad as too little.
If you invert my early language study habits, and take it as doing too little, I imagine you’d lose interest and eventually find it harder to gain fluency, and I think you can apply this too little/too much to virtually anything. Too much revision feels like cramming the words into your head, which I find sad.
Too little is like placing a butterfly on a flower and letting it fly away as it pleases.