In the 1970s, the Army taught us to only load 18rnds in the 20rnd mag and only 28 in the 30rnd one. This was because of concerns about "reliability".
Now, the Army (or the people in it) are s thought to be the experts but I have experienced the same kind of uninformed guidance there as one finds anywhere else. The main difference is that in the Army, you have to do it their way.
Leaving springs compressed leads to failure! That was basically true 100 years ago, especially when most gun springs were the flat type, which are most vulnerable to taking a "set". And it was true because knowing how to make a spring that wouldn't take a set, and actually doing it were much less common than today.
I have my grandfather's shotgun, bought new in 1909, and one of the advertised selling points was that its springs were guaranteed "never to take a set". Not just for the lifetime of the owner, never. And to this day, they never have. Sadly, the company backing the guarantee went away decades ago....
We know today that a properly made spring (and most are) does not weaken from being compressed and left compressed over a long time. IT is the cycles of compression/relaxation/compression that weakens a spring.
If a spring weakens only from being left compressed, its a "bad" spring. They do still happen, just not often these days.
All else being equal (and it almost never is) bigger bullets tend to work better.