There is lots of over weight people in many sports. Skiing is Athletic. If you check the calorie burn in an hour, in addition to regular calories burned, even for a light effort (not much effort just skiing down easy runs), it is 348 calories. This is average burn for going up and down chair lifts at a slower speed. Moderate effort if about 450/hour and advanced or racing (really fast skiiers, lots of runs per hour), is 611/hour. This again is average burn going up and down chair lifts account for. It uses all the large core muscles (quads and abs), which are the big calorie burners. As people have fun and significant amout of the calorie burn comes from the muscle used to stay balanced on the ski alot of people don't realize the effort they are putting in, but a couple hours onthe slope light skiing will burn more then a 45 minute aerobic work out. That's hard to beat. If you wish to check my numbers I use www.nutritiondata.com
for monitoring my daily intake and expenditures and find it highly accurate.
If you want to ski well you need to be in good shape or you can fake it and hurt for a few days as you will after a game of pick up foot ball or hockey if you don't do it always. I love to skate too, but being as I don't skate all the time Like I did when I was young I hurt a little for a few days if I get too agressive on the rink.
The skating from run to run, walking up hill to get to lifts and other exertion put out in the cold temperatures is also very athletic and often more effort then coming down the slope feels like.