As for size of Windows dir - it not "leftovers" from uninstalls (but they exist too of course), it is a feature of Windows and 95% of it is a Windows updates, all chain of updates is stored locally and can't be safely deleted (according to MS engineers). Yes, it grows very big, especially if start point was older version/SP of Windows. In extreme cases, e.g. installing release version of Win7 and then updating to all service packs and updates will cause it to increase by 15-20Gb. But it is a known expected "feature" and it doesn't slow your PC, only clutters your c:\ partition. And re-install will not reclaim this space, it will be taken again immediately as soon as updates arrive on your fresh install of Windows.
If by chance you a talking about clutter of 3rd party apps, then it is not a windows problem. E.g. some drivers like to keep older installers in appdata, some apps keeps backups of data there - worst offender is Garmin updater, which creates 12Gb backups every map update. That's not Windows issues.
Honestly, it's indefensible to have a 30GB Windows folder.
This isn't true.
It's routine for goofy conflicts even within MSFT's own stack (Office + Windows + whatever) to put you in a place where reinstalling Windows is the easiest fix.
Plus, if you install and uninstall lots of software (e.g., if you work at a software company), C:\Windows just gets bigger and bigger over time. The only way to reclaim that space is to wipe & reinstall.