My experience with the MK4 Garmin update would backup the file size issue
After my last Garmin update the MyMaps display shows data from 2021, 2022 and 2023
On the Garmin standalone units the whole database is replaced so MyMaps just shows the latest. I suspect at some point in the future the MK4 will become full and will need a full clear out just like the MK3 mentioned here
In the first step of the map update, the unit writes a folder on a pendrive (called Garmin) with a file (garmin device.xml) containing all info about the unit, including the avaiable memory size.
In the second step, this file read by GarminExpress app running on a pc is used to generate the proper set of update files, and I think that the avaiable memory size
should be considered. This does not happen because the unit, once reconnected the pendrive, says "not enough free space in the unit".
To be precise: EU2023 7,68GB, EU2024 8,41GB.