Your pic is also a perfect example of why competitive bodybuilders are the only people who should use bodybuilding poses for progress pics. To get mildly technical, your hand is supinated in that pose, which changes the contraction of the biceps and gives the arm a different appearance in pics.
Like we've all said, it certainly isn't something to try "fixing" and isn't really worth paying attention to. If your knuckles were facing forward in the same pose, that "gap" would magically fill in and you'd have nothing to get distracted by.
Little. And entirely in proportion to the rest of your body.