The bug

The sleeves of chestplates are designed for the traditional 4x4 pixel arm size, and do not fit the newer 3x4 arm size introduced in 1.8. I believe this to be a bug due to the fact that the second layer of skins do in fact adapt correctly to the 3x4 arm size rather than 4x4 - the chestplate sleeves should follow suit.

How to reproduce

1. Use the "slim" skin model
2. Equip a chestplate

Expected results

The chestplate sleeve would scale accordingly to fit the decreased arm size.

Actual results

It remains at the usual 4x4 size.

Further notes

While skeletons can also equip chestplates, with this resulting in an even bigger gap size, this makes logical sense due to the fact that skeletons are meant to have decayed and therefore have arms smaller than what is reasonable for players.