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.
Could you please explain where was the segment created? - The segment which I have created from Adobe Audience Manager
Did you create that within ACS or in AAM ? - From AAM I have created segments, post push from ACS I have configure on Audience category
Is there a profile merge rule on the AAM segment? - No Authenticated Profile + Device
Is it the declared ID based segment or the MID based? - It is on Recipient Marketing Cloud ID (MID)
Just For Example
If I have created one segments in AAM after 36 hours the Total population is 13,000. While I have shared the segments via Destination and push to Adobe Campaign Data sources and also configure from ACS after 36 hours the same segments (Audience Category from ACS) counts some what looks like 5,000.
My only query is why the count are different while pushing from AAM to ACS, it suppose to be same or some what near by right?