Replying to Noob powder question/help

I'm sort of new to skiing, and I can ride all over the mountain pretty well do 540's, and slide rails and stuff, but I seem to have a problem with powder. Whenever I would ride powder for extended periods of time last season my thighs would burn so bad, they would get so tired and I don't know why. And I'm in really good shape so that's not it. I have a few theories that could be the reason, my form is probably wrong, so what are some good pointers. Also could it be because my skis are too skinny(80 mm) and could it be that my boots are too big and soft? they were rossi soft 1s and I weigh 160 pounds, also they were about 1 size too large(I'm gonna go see a boot fitter and get new boots soon). Thanks for the help