I have a combo rice cooker/veggie steamer that I use so very often, mine is worn out from use of probably every other day, if not every day for the past several years and we will be getting another one (ours is a black & decker)
I'm just sharing that because we use that kitchen appliance A LOT so I dont steam in the microwave as much since that is my go-to whenever steaming. That said, I still will and do steam things in the microwave - especially if I'm already cooking rice (they came out with the double decker right after we got this one). Mostly broccoli though so I am not certain about carrots. I would guess that it shrivels because it is a slightly over cooked?? What I do when I steam my veggies is put it in a glass bowl, and the water used to rinse it off or a handful of water - just a splash, enough to create steam with, but not to cover and then have to pour out, cover with plastic wrap and nuke 'em - probably 2-3 mins at most??
One thing I have noticed that perhaps could be used as a guide to whether they're done or not is when the plastic wrap is tight, and the food is hot but not completely when the microwave stops and it sits for a little bit, the wrap wont stretch all the way down to the veggies. But when it is good and hot, then the plastic wrap often will stretch all the way down and mold itself to whatever is in the bowl. Make sense??? Of course with carrots, your best test of doneness is to poke it with a fork. Once soft, it's good to go.
So, experiement, try just a splash of water and less time.