A Google employee filed a bug report for Chromium, the basis behind Chrome, last month. The report pointed out a bug with Chrome that kept old versions of Chrome apps and extensions in the user profile. This means that with a few extensions that update frequently, you could begin to lose a significant amount of storage. The Google employee noticed the problem using the Google+ Photos app. It’s a fairly large app at 56MB and updates almost weekly. As a result, the extension used up over 700MB of hard drive space.
The scope of the bug report grew to find the problem with all apps and extensions across different operating systems.
For Windows Vista, 7, 8, and 8.1, you can see evidence of the problem here: C:Users[username]AppDataLocalGoogleChromeUser DataDefaultExtensions
Each extension is given a random name and below that you can see the older versions still hanging around. For other operating systems, you can see the Chromium User Data Directory for where to look.
The bug report resulted in a workaround being put in place. This doesn’t solve the underlying issue of old extensions being locked and unable to be deleted but it does mitigate the issue by unpacking new versions in a temp folder and should prevent old versions from piling up because they are in a folder that is synced with Google’s servers.
For the meantime, you can either (carefully) delete the older versions of extensions under your user profile or you can wait for the bug fix to make it out into a stable release and have it clean up your user profile.