hi!
FullAccess users is populated usually by filling the following conditions:
- Is user enabled on AzureAD?
- Does user has a valid license?
- Is user part of security group?
If the tenant has some particular licenses (PowerApps per app, Project, or certain M365), users might be synchronizing into the environment and therefore, showing on "Full Access users". That's why, using a Security Group allows you to control not only access to the environment but also the user synchronization process.
You can also open a support request to dig furthermore, and check why the increase in the number of users.
Regards