Hi Vikram
To change how long logs are kept for you can go to Site Administration > Server > Cleanup > keep logs for.
The default is never delete logs which can be reduced.
The following advice is given
"It is best to keep logs as long as possible, in case you need them, but if you have a very busy server and are experiencing performance problems, then you may want to lower the log lifetime. Values lower than 30 are not recommended because statistics may not work properly."
So you should only consider reducing the time to keep logs if there is a performance problem with Totara.
I don't know why your log files are so large, was this a large site that had stats enabled and has been going for several years?
regards
Craig