They have the same amount of flash like most other APs from MT have: 16Mb. They just have slower CPUs.
The big difference is that they have only 32MB of RAM, and hence no space for a RAMdisk.
In the other models with 16MB flash (which usually have 128MB or more RAM) the upgrade packages are downloaded into RAMdisk and the installation can take place in the 16MB flash without disturbance.
In the hAP lite and hAP mini, the packages are downloaded into flash (just like in the models with >16MB flash) and so there has to be space in the flash for:
- the existing installed RouterOS
- the newly downloaded upgrade packages
- the working space during installation
In practice that means that even an existing combined-package v6.xx install as it comes from the factory can only be upgraded for a limited number of times, until due to the always occurring space leakage there is no more room on the flash.
Indeed as mentioned you can escape the certain death by using a separate-package install and have only the packages required for your usage on the device, especially when that usage is only a simple access point and no advanced features.
But in v7 that is no longer possible, and v7 will be guaranteed problems.