Sure they would expand, just like a marshmallow does. Do your marshmallows get compressed when exposed to a vacuum? That would be very strange indeed. It's a much different situation with the bags as there is much less open space. A bag collapses close around the hops. That doesn't happen in a jar where the volume remains constant. I have about half of my hops in vacuum sealed jars right now. I would think that any degradation would happen slowly. I plan to continue using the jars until something indicates there might be a problem. I'll do some seat of the pants comparisons, but with so many variable and much relying on subjective impressions, the results probably won't be at all definitive. Don't get me wrong; I like the jars, but seems that we might be protecting the hops from oxidation at a cost in a reduction in aromatic oil utilization. Hopefully it makes no noticeable difference one way or the other.