When you define a remarketing audience, Analytics adds users to that audience based on the criteria you choose. When you share that audience with Google Ads, Google Ads then has a list of users who become available to see your remarketing ads.
When Analytics applies sampling in order to increase the speed of a report query, that sampling has no effect on remarketing audiences. In this case, Analytics has all user data. The information in reports is a sample of that entire data set, but the audience populates from all of the data.
However, if you apply sampling via _setSampleRate (ga.js or dc.js) or sampleRate (analytics.js), then some percentage of users is necessarily excluded from your Analytics data, and thereby from the possibility of being included in your remarketing audiences.
In order to provide optimal processing for remarketing audiences (remarketing lists in Google Ads), Analytics creates two classes of audiences, and processes hit data for them either in real time or once per day. Analytics processes remarketing audiences at the property level, so all of the audiences in a property are processed on the same schedule.
A 180-day grace period during which all remarketing audiences in an Analytics property are processed in real time begins when you enable Remarketing.
When you create a new remarketing audience, a 180-day grace period begins during which all audiences in an Analytics property are processed in real time.
If an Analytics property includes at least one remarketing audience that is currently targeted in an active Google Ads campaign, then all remarketing audiences for the property are processed in real time.
There is also a 180-day grace period during which all remarketing audiences in a property are processed in real time that begins with the last time an audience was targeted in an active Google Ads campaign.
When none of the audiences in a property have been targeted in an active Google Ads campaign for more than 180 days, then Analytics processes those audiences once a day.
How processing affects list size
Audiences used as remarketing lists for search ads and audiences used in Display advertising are subject to different conditions under which they can accumulate users, and can thus have noticeably different numbers of users.
Remarketing audiences used in your Display campaigns are not subject to the same privacy restrictions, and they accumulate users as expected regardless of whether they are processed in real time or daily.