Originally Posted by FBinNY

Now that everybody has weighed in, let me toss a con wrench into the mix.

I suggest folks look at the photo, and read the question more carefully.

First of all the, at the risk of being called Goldilocks, the chain is neither too long, nor too short, and is just right as it is. Note that the RD has moor travel room in either direction, so there's no reason to make any change.

Next, the chain sag isn't the result of chain length but is a normal artifact of changing spring tension as the cage rotated through it's arc. As the chain takes up slack with smaller sprockets, the spring gets more relaxed, and the chain tension. ALL derailleurs do this, and it's a progressive effect, with more tension with bigger sprockets and lower tension with smaller.

The amount of tension and how much it changes is a function of the derailleur's design, relating to the spring used, and the number of turns it has. Odds are that the chain always sagged this way, but the OP never noticed it because he never had a reason to. (and still doesn't).

As for any change being related to chain stretch (wear), consider that we consider chains toast long before they "stretch" 1/2" overall, no exactly an earth shattering factor.

So, to the OP, everything is OK, continue checking your chain for wear, keep it lubed and ride your bike instead of obsessing over every observed anomally.