Proportion refers to the relative size and scale (ratio) of the various elements in a work of art. This creates a relationship between objects, or parts, of a whole particularly in relation to size or quantity.
Differing proportions within a composition can help establish visual weight and depth.
Where one element appears to be larger than smaller than we expect it to (for example the size of a hand compared to the size of the rest of the body) it appears to be out of proportion.
If you are interested in knowing more about Da Vinci, you may wish to watch this BBC video