As@6x6 surmises you probably don't have the full macOS installer downloaded. macOS many times will just download a small 20MB stub installer which then grabs the full 12GB installer. You just need to make sure to quit the installer once the full installer has been download to make sure it doesn't try to automatically upgrade your system (it shouldn't, but sometimes I've seen/heard it happen).
Another option is to use the Terminal command line to download the installer. Currently I think the 12.7.6 installer is not available, but you should be able to grab the 12.7.4 installer using the following command:
softwareupdate -d --fetch-full-installer --full-installer-version 12.7.4
If you want to see which versions of macOS you can download this way, you can use the following command:
softwareupdate --list-full-installers
FYI, you may first need to delete the current stub installer.