Spicy mustards, arugula and kale make up the light weight category. They are 'lofty' and add texture, color and interest to salad mixes. Often, these are the greens that you don't want too much of in a salad anyway. I've tried the mainly-mustard-greens salad and it was too much. The keyword there was 'tried.' Other varieties of kale may produce a heavier yield, but the russian kales have rather spindly leaves when they are young.
What would you choose to put in your salad mix?