Oops, I meant that the SA convinced me to size down, not up. I fixed my original comment.
But yes, me too -- sometimes tight pants, if they slide downward at all, squeeze the flesh upwards like toothpaste and give me extra muffin top. High-rise jeans that are loose in the seat and legs are the only jeans that don't budge, but they look god-awful -- exactly like the SNL video. No one wants a long, flat bottom!
Janet, I think the key is "a little ease in the fit", like you say. Too big can indeed make one look bigger (even on small women). I find, on me, this is especially true of boxy, baggy tops. If you can't discern the shape of my bust, it just looks like a big block and is unflattering. I can wear oversize tops if they drape nicely, though. I also find that if something is a little loose and boxy, adding a long pendant necklace creates a "valley" between my boobs and solves the problem.
I think we're all a little cautious of sizing up on the part of our bodies that we feel is a little bigger than we'd like.