Do I not understand the flex-grow property?

Question

I\'m afraid I must not understand flex-grow. If you jump to the JSFiddle below - the way I understand it, .big should be three times the size of the other

Answer 1

Authors are encouraged to control flexibility using the flex shorthand rather than with flex-grow directly, as the shorthand correctly resets any unspecified components to accommodate common uses.

https://drafts.csswg.org/css-flexbox/#propdef-flex-grow

.flex-item {
   flex: 1;
}
.big {
   flex: 3;
}

This is a working example

http://codepen.io/anon/pen/VjZYPV