Actually, when Clarice examined the dead girl in the local mortuary earlier on, one of the things she noticed was two unexplained diamond shaped areas cut out of the back of that girl, in addition to the gunshot wound made at close range (muzzle stamp on the skin) and a death's head moth (or pupae, I can't remember which) stuck, or shoved down, the throat of the dead girl.
Later on, after reviewing the case notes she realizes the fact that only the first girl killed was weighed down (third girl discovered), she and her study partner/roomate surmise that Buffalo Bill must have known his first victim. She then travels to Bimmel's hometown and goes to her house. Looking through the victims room, left in fairly pristine condition as a "shrine to the dead", she sees a house clearly through the window and realizes that not only was she known to BB, he watched her. She continues to look around the room and sees that Bimmel was a seamstress, making her own dresses, and further notes the uncompleted dress on the seamstress' form. It, too, has the diamond shaped patterns pinned to it.
It would be safe to surmise (plus I seem to recall that Clarice finds nudie pictures in a jewelry box) that Bimmel had gained weight/was now largish. When BBill takes his victims they are all largish (not 'big boned' but plump) (we know it is because Bill is forceably causing them to lose weight and having them use skin lotions to make their skin more pliable and easier to work with). When you skin something, beit an animal or, so I assume (I've never done it...:) a human, and treat the skin to be used as materiel to make clothing (I've done this with deerhide), it will shrink. When Clarice sees the dress on the dressmaker's form, the diamond shapes of material pinned to the back are exactly like what she saw missing from the girl in the mortuary. (Those pieces, in that shape, I assume, are a way that a dressmaker might plus up the size of the top of a dress.)
Now Clarice, who has proven to be extremely intuitive, puts all of the minor clues together and now knows that:
- BBill knew his first victim and watched her.
- Learned how to modify the size by watching Bimmel.
- Is skinning the victims to make a 'dress' out of the skin
- Is trying to 'become' a woman (as we know, the tooth fairy before 'became' something else)
- She could then track down BBill's whereabouts by speaking with the current owner of the house in back
She attempts to call her mentor, but he thinks he's found BBill and puts her off. To clean up the records she continues on her interviews anyway and goes to the house where BBill might have lived, not knowing that is where he (still) lives. While waiting for the man to get the previous occupant's address, she sees sewing materiels, and more importantly, a death's head moth flitting around.
Now she knows the man in front of her is actually BBill himself, at which point, as she is (poorly) drawing her weapon, he slides into the back, heading downstairs.