Understanding the mecahnics of an erection might better help you understand the answer. An erection is limited by three things, the size of the tubes in your penis that hold the blood during an erection, the amount of blood that enters them and the amount of counter preseesure coming from the outside. A change in any of these will affect the size of your erection.
The size of the tubes is not going to change unless there is an injury, so that's not the problem. (It's also the reason why there is no possible way to increase the size of your penis).
As we get older it's natural for the amount of blood flow to decrease. This is due to a build up of plaque in the arteries leading to it as well as a weakening of the arterial walls. This is a perfeclt normal part of aging. It's also why when we're younger our erections point up at the ceiling and later they stick straight out. (This is called the erectile angle). If the decrease is severe enough there is an inability to get an erection (called Erectile Dysfunction) and medications such as Viagra or Cialis help to increase the flow.
That leaves us with counterpressure. As blood enters your penis it exerts an outward pressure. Surrounding air pressure pushes back and when the pressures equalize it can't grow any more. If you decrease the outside pressure more pressure can be exerted and it will grow bigger. (That's why you will have a larger penis at higher altitudes). Because of this penis pumps can increase your size temporarily. They create a vacuum around it allowing for an increase in penile pressure. This can help you get a larger erection but only for a minute or two. When you remove it the outside pressure will increase and they will equalize.
Not the answer you wanted to hear but those are the facts.
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