I think it'd be nice if the "Total Installed Size" was the incremental size between the (after upgrade) and the (before upgrade). Since this information is presumably being displayed primarily to let the user know how much hard drive space is about to be used, it's only the additional space that the user would care about.

I almost wonder if "Additional Installed Size" would then be the more appropriate string to use. And I'm not sure what you want to do if the incremental size is negative (e.g. the infrequent situation where a newer package might be smaller than the currently installed one). Show the negative number? Show the "Remove Size" or whatever the string is?

Just throwing out the idea(s).