OSX update cache: does it accumulate over time?
I suggest you download a utility that shows the size and arrangement of your files. Personally, I use DaisyDisk. It is shareware, so for your purposes, it would suffice to trial the software.

Mail attachments are stored in ~/Library/Mail Downloads/

It is unlikely that OS X is keeping packages after installation through Software Update. You should first look to your cache ~/Library/Caches/ and perhaps your sleepimage, which is located under /var/vm/sleepimage (note that the sleepimage is recreated every time the system goes to sleep. Read more about disabling it here if you are concerned with HD space).

DaisyDisk (or equivalent) will do a much better job at identifying where the "growth" has occured on your filesystem so that you may excise it than we ever could. I suggest you give it a try.