I have been using PIWIK for some years now, starting somewhere around version 1.7.
I just compared the filesizes of the piwik.js file, and it has increased from around 15kb (vers.1.8) to now around 24kb - which is still amazingly slim for the load of functionality offered.

Since there will propably even more features added in the future of PIWIK, it would not be surprising to see the filesize grow and grow, which might at some point turn into a nuisance, especially for those using PIWIK to track mobile devices.

From my point of view, it would be very beneficial if only those segments of the "core" would be transferred which are going to get used.
Not every website will need the ecommerce tracking, and many small websites may not even do any goal/conversion tracking.
The whole geolocation thing is out of bounds on shared hosting where a user cannot install the necessary ad-ons, and the newly introduced content tracking feature will propably not be needed by hobby programmers.

Since the piwik.js-file is fed to the visitors via the index.php-page, maybe it is possible to reduce the size of the file to transfer by assembling a custom js that filters away all the "unnecessary" code.
A simple cross-check with the plug-in-enabled setting chosen by the PIWIK admin might be all that is needed to find out which modules to serve.

Another approach on this issue might be a general what is "core" and what is "external" re-thinking.
If adding plugins will be made easier thru #6106, maybe the core can be slimmed a little for those just wanting to know if they have some visitors at all :)