Layering makes for fantastic athletic fashion. It works well for plus-sizes because it allows you to wear trendy workout pieces, while still maintaining coverage over your curvy figure.
You might try experimenting with belly-baring tops and high-waisted workout pants under a baggy, airy tank. Or you can do a form-fitting tee with yoga pants. You might try wearing a colorful sports bra and a thin, white top and matching running pants. Choose outfits according to your body type and layer accordingly.
Of course, it's important to note that you don't want to layer too much because as you work out and sweat, you should be able to shed some of your pieces, if necessary.