How can I check if an available update of a given app that I already have installed will take up more or less storage space than the one currently installed from the information in the play.google.com page? Is there a URL I can use to check the size of my current version and the one of the available update? Can I mount the device and list the size of the installed apps and compare it to the advertised size in play.google.com?
For example, I updated Gmail in the last few months, and originally took up 2.07MB Application size, and the last two updates went from 3.08MB Application size to 3.26MB version 2.3.6 (20110305). How can I know this in advance?

