Why are my hourly 'Time Machine' backups so HUGE!

I recently purchased an external hard drive dedicated to being my time machine backup drive. I found out that it backs up any changes you may have made every hour. When i hold my mouse over the little spinning clock logo during an hourly update, its not unusual for it to say “backing up 2.* out of 23 gigabytes!” there’s no WAY i made THAT many changes, most of the time im simply browsing the internet! is this normal? if not, what do i change to stop these HUGE hourly backups from clogging my drive. the drive will be full everyday at this rate!