Interesting question. Since you are using device profile only to qualify for your segment, the segment population being high isn't surprising. Suppose you're qualifying into a segment based on pageview on your homepage. All your homepage visitors get qualified into it (all have MID also, since visitor service would be active on your homepage). Now your segment let's say has 10,000 qualifications. If you export this segment, Adobe Campaign won't necessarily have contact information/mapping table for all MIDs. I believe that's the reason why you're seeing such a big mismatch.