Capacity - Part 1 - Whats That About?

For all you car lovers out there...

You understand all about an engine's capacity. A 1300cc performs very differently to a 4l V8!
Capacity is kinda obvious when we are talking about cars... but I've included the dictionary definition (some of it) anyways:

This is dictionary.com's definition of capacity:
American Heritage Dictionary - (http://dictionary.reference.com/)

1. 1. Ability to perform or produce; capability.
   2. The maximum or optimum amount that can be produced: factories operating below capacity.
2. Innate potential for growth, development, or accomplishment; faculty.

So understanding a cars capacity is fairly simple...
But what about when we are talking about human beings?

Are we like car engine's ... given / born with a certain capacity...
or is our capacity virtually limitless?

How do we know when we are reaching our capacity?
Are we able to move from being a 1300cc to being a 1600cc and then an 1800 and then a v6 and then a v8?

What are your thoughts?