Segments with merge rules that contain device graphs e.g. Last authenticated + Profile link device graph are only evaluated in DCS (realtime). You can find this in the frequently asked questions for profile merge rules: https://marketing.adobe.com/resources/help/en_US/aam/profile-merge-faq.html 
For total segment population, we compute the number of unique devices that are members of those segments. However, those segments are only evaluated in realtime , so users must be seen in DCS in order to be qualified for those segments. This explains the low segment population for impacted segments.
If I build a segment with a Profile Merge Rule that uses a device graph and the segment is using both real-time and on-boarded data, will my segment be updated as the on-boarded data changes?
No. Currently, Audience Manager evaluates segments with a Profile Merge Rule that uses a device graph in real-time only. Updates made to on-boarded traits after the segment has been evaluated will be used to qualify the segment when the device is next seen by our edge data servers. This assumes the device profile is still active in the edge servers and the on-boarded data has been made available to those systems. See also, the Privacy and Data Retention FAQ.
Which devices can qualify for a segment using a Profile Merge Rule that contains a device graph?
To qualify for a segment, devices must have been seen by Audience Manager on our edge data servers after the segment was created.
We released an update to our Profile Merge Rules in late July (email attached), that impacts how Audience Manager segments evaluate data with device graphs. As a result, onboarded segments that use the device graph are impacted because the graph is now only evaluating realtime data.
As per documentation , in order to increase the reach of customer campaign we would recommend to use profile merge:
Last Authenticated Profile + Current Device Profile
Update on your recommendation:
In actual the Profile Link Device Graph will have greater reach as it contains current and last 3 device profiles. It is the reporting that shows the real time activity and going forward device activities in this Profile Merge rule, that is why it shows low in reporting.