hi,
run the following commands.
1.dbcc sqlperf(logspace) . in the out put u can get the log size.
2.backup log <dbname> with truncate_only
3.dbcc shrinkdatabase(dbname,10)
4.dbcc shrinkfile(2) (2 means log file)
after these steps over execute step 1 again after 10 or 20 mins ,let me know the log size growth.
good luck
Bharat