You are probably using the printer driver that shipped with the LaserWriter Pro 630. This software included different print driver files for different configurations, and you have probably selected the wrong one.
Although you could reinstall the correct printer-driver file as described in the manual, you should upgrade to the latest version of LaserWriter driver for Windows 3.1 that Apple has posted online. This software now includes many new features, including the ability to switch between 300dpi and 600dpi.