I've seen that Android automatically generates a thumbnail when taking a photo, and saves it as an extra with the key "data". Is it possible to disable that action, as to save space in the device?
I've seen that Android automatically generates a thumbnail when taking a photo, and saves it as an extra with the key "data".
No, it does not. Camera apps may generate thumbnails and put them in a data extra in the Intent used with setResult(). Android, the operating system, does not.
Is it possible to disable that action, as to save space in the device?
I do not know what "space" you think you will "save". That thumbnail is held briefly in system RAM. It is not stored on disk.
If you are a programmer, and if you are writing an Android app, and if you are using an ACTION_IMAGE_CAPTURE Intent with startActivityForResult() to invoke a third-party camera app, then you can include EXTRA_OUTPUT to indicate where you want a full-sized image to be written. According to the ACTION_IMAGE_CAPTURE specification, the data extra is not needed in that case. Some camera apps will then skip that extra.
However, developers of third-party camera apps are welcome to do whatever they want. If they want to create the data extra on every ACTION_IMAGE_CAPTURE request, that is their decision to make, as they are the ones writing the camera apps.