There isn't any 'one rule applies to all' when it comes to updates, just too many variables involved. Some updates are just minor bug fixes (maybe a few library files that are only KBs), some are significant alterations (a change to the user interface that are several MBs), and some require the app to be completely re-written. Plus some of those updates might result in the space the app requires to decrease but often will be an increase. This can vary from app to app along with what the purpose of the update itself, a security patch might be really insignificant in size, a version upgrade might be much more so, but again, either one is big variable.