It's happened a few times before that Piwik ZIP package grows by Megabytes, eg. Piwik 2.14.0-b4 package is 30Mb big instead of 14Mb #8144

The goal of this issue is to a new automated test that will count the total size of all files in Piwik codebase (including vendors/ composer packages), and fail when the total size is greater than what we reasonably expect (eg. current total size + 10% growth margin)

This is important because we want all Piwik users to upgrade to latest version and many users have slow connection between their servers and builds.piwik.org eg #7280. Therefore each Mb in the ZIP package can add minutes of waiting time for users to wait for Piwik to auto upgrade. Also Mb saved on package size means more people can actually successfully upgrade Piwik as they're less likely to reach network or other timeouts.