I have, and that's how I came to my conclusion.
I just think certain people have more myostatin than others. Myostatin is like your body's muscle-building stopping point, so it inhibits the ability of the body to easily put on muscle and more importantly, to retain that muscle after a period of time of doing nothing. That's why they say that PED usage is generally set up in six-week cycles. It's not because the drugs just magically stop working after six weeks. It's because mystotain levels increase each week until they get to the point six weeks later where the body says, "Nope. Your muscles aren't allowed to get bigger than this".
Well, if you arrived at your current volume and frequency by experimentation (trial and error), then that's all you can do.
I'm vaguely familiar with myostatin, as I've read a bit about it online and McGuff covers it in BBS. If you're an ectomorph, then I'm afraid to ask what I am.
Interestingly, I've recently posted in another thread that I went from 3x/2wks to twice a week at the beginning of the month. And about midway I added 3 additional compound sets for a total of 10, excluding calf raises and HIIT cardio. Since that time, I actually lost a bit of size in my chest, arms and legs. (I measured on Thursday.) Very slightly, but I can't attribute it to measurement error. And so, I'm now debating whether to reduce frequency back to 3x/2wks or possibly even once a week again, with another very brief HIIT session (~6 minutes) thrown in midweek.
As I've noted in the past, I was fairly bigger when I was doing a lot more volume with either an ABA/BAB routine (to 2005) or full body twice a week (from 2005 to 2012). But by the spring of 2012, as I was approaching my mid-50s, I could no longer keep it up. Which is why I started experimenting with lower volume at first, and then with reduced frequency as well. And, yeah, I gradually got smaller. But was it because of the change in routine, or the inevitable result of getting older? If I had to bet a dollar, I'd say the getting older part had a bigger hand.
Regardless, I could no longer handle the volumes I had grown accustomed to earlier in life. Plus, an accident outside the gym in the spring of 2013 left me with complete full thickness tears of the supraspinatus and infraspinatus tendons of my left shoulder, along with a partial substance tear along the long of the biceps. My right shoulder, the "good one," already had a full thickness tear of the supraspinatus tendon, and split tears of the subscapularis and biceps tendons. Those were the result of bad exercise form in my earlier years. So I'm guessing my physique is a bit compromised and took a bit of a hit because of the weakening effect of those injuries, notably the one in 2013, although I think I work around them very well.
As a final point, I believe I posted a link some time ago about an interesting exchange regarding workout frequency between a fairly young lifelong natural lifter and the guy who runs the ExRx website. Have a look:
https://exrx.net/Questions/TrainingFrequency
And if you ever do decide to read BBS, let us know what you think.