I wrote a tip about Adobe applications, to which you can add iTunes. If you open iTunes, like I do, shortly after rebooting or logging in, it will sit there pretty much using just the space it needs. However, if you use iTunes, especially connected to your iPod, iPhone or iPad, to synchronize them with updates to applications or for the backup copy, you will find the space increase significantly.
If you use iTunes to access the iTunes store, you will find the space will increase signficantly. If you play songs and especially video files in iTunes, you will find the space really increase. In the end iTunes will easily increase by a factor of at least 2, often 3-5 times its original size.
The answer is the easy thing, close it, use terminal mode with the purge command, wait for the command to finish as it freezes your computer for a moment, and then reopen iTunes and roll on. You can open the Activity Monitor app to see what the applications are doing. I keep this application open (hidden) all the time.
Note about the purge command. Use it when you're not running Time Machine backups. It doesn't interfer with the backup, only slows the purge command to determine which space the backup has and won't use again to remove it. I just wait until no application is doing anything to run the command.
Another tip with iTunes, is don't connect any remote device either direction or through wifi until iTunes is fully open, and maybe like me, scroll the list of albums to get all the covers loaded. I don't understand why iTunes doesn't load all the albums covers when you start it.
Well, I do, to save space and load time, but on any Mac anymore, that's useless as they all are fast and powerful enough to handle loading all the album covers. I always scroll the list 1100 albums so I see all the covers all the time to find the one I want. I'm a visual person so I remember the album cover than the name.
The other option than the purge command is just wait for the daily maintenance which runs in the early hours based on the time stamp on your Mac, not where you're at if your travelling. This automatically recovers unused inactive space left by closed applications.
This is where the Activity Monitor application is useful, to see what inactive and if you're approaching the limit of your cpu memory, see if the Mac is swapping space between applications and files. Both the page out and swap statistics should be low if not zero.
Anyway, that's just an obvious Mac tip.