Although Time Machine only backs up changed items in its hourly backups, since it backs up the entire drive, it also includes cache files which change frequently.
If you use VMware or Parallels, your virtual disk images are several gigabytes and will change continuously as long as the VM is running. That can take up a lot of space in your Time Machine backups and can take a long time to back up. To avoid that, exclude your VM folder in Time Machine options. You should still back up your VMs manually, though.
Time Machine does not include caches by default. There is a bit of metadata to flag files and folders to exclude them from backups.
Smart applications (watch out for the next releases of programs that require monolithic files, like Fusion) will flag their files as “do not back up” automatically.