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It's been a while since I have been interested in commenting on a question - but I took this question to mean something different than the answers given thus far...if I am wrong, delete and move on!
If you look at what the commercial industry is doing in general with software development, they are moving towards the "appliance" phenomena - that means, tightly integrated software solutions that are small and fast. in many cases, can stand alone as a complete solution. When I saw "slim or fat" this was my thoughts behind the question. I am a developer and solutions provider for large banking and financial based commercial applications.
Slim can be looked at in a couple of ways - does it mean low on feature set? Small in footprint? Fast on execution? I think that if a "Slim" approach is the direction, you have to look at the overhead involved in the amount of code required to complete a task. Simply put, the number of iterations of code required to perform a task.
I am sure that if all code was reviewed in any solution, you might find complete blocks of code that are many years old and still performing a job. And over time those blocks have been modified, and to a certain extent bloated beyond original recognition. This is how code gets "fat." Developers are too lazy to rewrite - they just add on.
It's not a bad idea to simply throw away code from time to time, and recreate! This can be applied to a database table approach as well.
How many times does a module that relies on some database transaction get modified to the point that the original table structure is no longer recognizable or efficient? Database rewrites, although painful, can produce a "slimming" effect on code. By gaining efficiencies where table space is either no longer needed, or no longer being used in the way it was intended, you may find ways to maximize return on code.
The bottom line is, don't be concerned with what your competition may be doing, but rather concerned with how you can maximize the strengths of what you have or need to produce. In the case of many of these CMS's, you are relying on an interpreted code that relies on a compiled object based tied binarily to a specific O/S and chipset on a server of some sort. That in and of itself is inefficient. Sometimes "slim" does not equal fast, and "fat" does not equal inefficient...just continue to challenge the way development has been done and innovate!
Good Luck!