If you are already running an earlier version of Sierra (like 10.12.5), you can use smaller update files:
Because macOS Sierra is an older operating system (released in 2016), Apple has removed it from the standard "Software Update" section in System Preferences. However, it is still available, but hidden, on Apple's servers. macos sierra 10.12.6 download
For a clean install or creating a bootable USB, you can download the macOS Sierra Disk Image directly from Apple’s servers. If you are already running an earlier version