As compared to stars, planets are much closer to us. That is why they appear bigger.
Light coming from planet is like a beam of light, coming from cluster of point like sources of light (a collection of large no. of point sized sources of light). Here resultant variation in the amount of light entering our eyes from all the individual and independent pointlike sources comes out to be zero on average, Hence twinkling effects of individual sources are getting nullified. That is why planets do not twinkle.