Backups And Transaction Logs

Have a small shop and a maintenance plan that does a full backup on all user databases every night. I would have thought that a full backup (transaction log, too), would reduce the size of the transaction logs? Isn't that the point of a full backup? Mine seem to be growing larger. I do the shrink, rebuild indexes, backup the databse, backup the transaction log, clear history and folders older than 7 days. For example, the following db decreased only once during the period. But, note how the transaction log size continues to increases most of the time.

db, trn
253,196 - 394,245
253,193 - 365,695
253,191 - 360,572
254,219 - 381,699
254,223 - 406,346
254,223 - 411,018

Can someone explain this to me? Am I doing something wrong or just not understanding the process correctly?

Thanks,
Janet