I bought a few colors this winter and I find the Tippi Sweaters to be TTS - I am 33C/27/35 and an XS fits, but I prefer to size up to a S for a slightly looser fit. I can get self-conscious about my shoulders and arms being so defined in the XS, so I size up in this sweater. I even have it in a couple Mediums to do an oversized sweater look with leggings/skinny jeans, but I'm finding that I prefer the S.
IMHO, I find J.Crew sizing to vary from shirts to pants to coats. In knit shirts, I tend to wear XS-S depending on the style. Ex. if it's supposed to be "slim-fit" like some of the stripped Henleys, this past winter, I prefer to go with S and can even pull off a M (abet oversized). I found their bottoms to run large depending on the style: Cafe Capri's to run large on me, as I'm normally a 27 in denim, but can take a 00-0 depending on the fabric. Minnies on the other hand, are definitely TTS as I'm squeezing into a 4. For coats, people tend to say the mostly run a little large (but feel free to correct me if I'm wrong). I have the Schoolboy blazer in 0 and as long as I don't wear anything bulky under it, it's great. I don't shop at Loft, so I can't compare it to anything there, but I HTH!