If you are not having any issues, then I would recommend stopping your Lansweeper service, deleting the errorlog.txt, and restarting your service. A new errorlog will be generated when the service starts.
The cause of your large errorlog might be because you have debug options still enabled, or just because you have been using Lansweeper for a long time. You can revert any additional logging options by opening Program Files (x86)\Lansweeper\Tools\ConfigEditor.exe and use the 'Reset to default' button. After resetting use the 'Save configs and restart service' button.