I was in your same predicament when I started riding 2+ years ago, had just lost 120 lb. ( from 315 to 195!) Now, I find myself in the reverse situation. Gained back 30-35 lb. this last year. Had to size up when buying a new pair of Burton SB pants. Waist is fine, but because its a mid fit, godam seat n thighs are so snug, I'm not sure I could wear a base layer underneath.
(....I have been told "it makes my Ass look good!")
Until you settle into a steady weight, you r probably better off trying stuff on. Once you find a brand with a sizing system that worksfor you, then u should be alright ordering online. ...BTW, I decided to keep the tighties. Figure it will help motivate me to drop some of what I've gained back!
(...if that doesn't work, I guess this creepy Ol' Fart will be shakin' his tight ass booty all over S.E. MI. This season!) lol!