Shortening the chain will not make any difference in the small/small combination (but might cause problems for big/big, though not likely)

Based on the photo, the RD is taking up the slack, but you can confirm by slowly backpedaling as you derail the chain at the bottom to create still more slack. If the RD cage can wind back farther to take some of that slack before hitting the stop, then it's a pass and you can leave things as they are.

For a more detailed consideration, sprockets affect chain length by an amount equal to half the difference in the number of teeth. So going from 12 to 11t would "lengthen" the chain by 1/4", something that would only matter if you were already at the absolute limit for longest possible chain.

The reason you're seeing more chain slap, or close pass above the chainstay is simply because the radius of the 11t sprocket is smaller, and the chain leaves it barely high enough to clear the stay.