At least for me, the issue is that the stuff I put in the ziplocks (typically a full day's food in a gallon ziplock) has a tendency to push outward. The bags are not entirely airtight, especially as time goes on and holes get poked, so over time they allow the bag to expand, and pretty soon they have expanded to fill all available space. Especially true if you have 2-3 weeks worth of food in 14-21 ziplocks. Then, when you go to re-compress, you are dealing with 14-21 balloons that are slow to bleed air out of. I'd imagine it happens with anything compressible, but I don't use ziplocks much anymore.
