With creek boats "too big" usually isn't the concern. You are at the bottom end of the weight range for the medium and the middle of the weight range for the small. The length of the boats isn't tremendously different, but the medium has much higher volume than the small... around 40L or so.
Things to consider,
- do you feel comfortable in the boat? Some boats are too large because smaller paddlers catch their knuckles on the edges of boats when paddling or 'feel' like the boat is too big.
- where are you going to paddle? Narrow steep creeks? You may enjoy a smaller boat. High volume rivers, bigger is better.
Larger creek boats tend to be slightly faster, punch holes better, re-surface quicker, offer more of a cadillac ride with space to spare. On the flipside, smaller boats help in narrow creeks and can be more playful. If you can sit in the boat and see how it feels to paddle, my guess is you would do fine in either and if you have a good deal on a medium, go for it.