If you install a programme on a different drive it can still impact the size of the windows folder. It might be worth checking your winsxs folder size, which seems to grow at an uncontrollable rate whenever the OS is changed. I recently reinstalled my OS (in the last two weeks) and the winsxs folder is already 6.23GB . I’ve seen it grow to 12GB before. The bad news is that there is little you can do to stop it growing. On Vista it was possible to clean useless files in the winsxs folder from previous service pack updates with a special hotfix app, but other than that deleting files is highly risky and unadvisable. On high capacity HDD drives the size of the winsxs is not a problem but with SSD the size of that folder is expensive and can soon make fitting the OS on a small capacity ssd a problem if you have a few other apps on it.
There is a bit about the winsxs file here:
Although that link is for Vista it's more or less the same with Win 7.