Yeah, so this is a nuanced discussion. So dependant on a fighter's skill with such attacks, we can find a variety of things to be different. King Piccolo is not the most skilled ki artist, this is the only being in the Dragonball universe who needs to reduce his life span to reach his full power. To make him the end all be all of ki amp is not the right path. But that doesn't mean we discard what's going on here.
Just because a fighter CAN use a full power blast at a greatly reduced level of health, doesn't mean every fighter can.
SSJ Vegeta was severely depleted by Android 19 and despite being far weaker than Gero and Piccolo at the time, Vegeta is able to produce a Big Bang Attack that completely dwarfs the two of them.
SSJ Vegeta utilized a full power BBA on Android 19 despite his weakened condition.
SSJ2 Gohan also performs in the same manner. He's severely depleted here, yet right off the bat he is able to match SPC's FP Kamehameha, which is a solar system buster.
The only reason Gohan isn't winning is because he's holding back, not because he needs to get angry again. Goku urges him to "Release it Gohan!" which is unleashing his full power blast. For some reason, Gohan out of nowhere becomes overly concerned with the condition of the planet. But yeah, Gohan's blast is FP, despite never recovering any ki as well as Vegeta.