It is important to understand that narrowing the target group of your campaign by using custom segments will affect the sample size. The more specific your target group is, the fewer data points will form the basis for the reporting of your campaign.
For instance, a target group consisting of "young women who own a car and love McDonald’s" will result in a smaller sample size than a target group of "young women who own a car", which again will result in a smaller sample size than a target group defined as just "young women". It is clear that a campaign has a higher chance to reach an audience made of just young women than a specific target of young women who not only own a car but also love McDonald's.
Mind that if the sample size of your campaign becomes too small on AudienceReport, you will get a warning when editing the target group.