Not sure I understand the problem exactly.
If you are not using side tensioners, this would cause the quilt to measure smaller across the middle when you are finished. However, this would be a horizontal measurement, not vertical. Also, since you mention 4 poles pulling on the quilt, I assume two of them are side tensioners.
If the shortness is in the middle running vertically, the only cause I can think of is that the poles are warping in the middle from the pressure put on them. How thick are your stainless steel poles? How long are they? And where did you get them? And what frame are they mounted on? I have 10' poles purchased from the hardware store mounted on a Hinterberg stretch frame (which is a very sturdy wood frame). I'd have to check with my husband, but I think they are galvanized steel, not stainless steel. I'd have to look up the inner and outer diameters to be accurate, but the outside diameter is something close to 1-3/4". The longer the poles, the more they are likely to warp in the middle from rolling a quilt. Also, the smaller the diameter of the poles, the more likely they are to warp. This is what makes me really curious to know where you got the poles.
It's true that you do not want to have a quilt too tightly tensioned in the frame. A rule-of-thumb is to be able to poke a finger up from underneath the quilt layers and be able to grasp that finger with the other hand. It's kind of a "floppy" tension. In other words, you do not want the quilt drum-tight. Even the side tensioners should not be pulled too taut.
You may have a combination of problems -- weak poles that bend in the middle as you roll the quilt and too-tight tensioning.