What exactly is this "fat pig" statement based on? It's a bold statement but I don't think its really supported by any fact.
The weight of the UI primarily and system services that seem to get in the way when I could use the cycles for other things. I've had the same Macs through several iterations of OS X and I can feel the difference. This is my experience and not an attempt to offend anyone. I don't see Apple doing the same things as MS to make the OS feel light.
I have been testing different variants of OS's for the better part of 2 decades. Part of my own desire to be able to have objective hands on with them, and also part of work at various IT and software places.
Where OSx, in Lion and Prior tended to be very lightweight, and very linux like in it's overall requirements, Especially compared to fairly high requirements of early day windows. Today the opposite is the truth.
Upon several systems, OSx Mountain Lion started this trend of requiring more resources at boot time than previous generations. Mavericks was the all time peak so far for sheer base resources taken on fresh boot. (over 2 gb of RAM for example on fresh install post boot). Yosemite has been a little better on RAM usage, but slightly higher on idle CPU usage.
In Comparison, right now in the Windows and Linux camps, Both of these Operating systems can fully load on boot (fresh install as well) with well under 1GB of in use system memory.
While OSx on disk on fresh install is still much lower than Windows, there are things it is doing which causes CPU usage to be higher on average than windows.
This doesn't take into account other things such as gaming performance, which due to outdated drivers that are slower to update, OSx does not compete with the pure performance on the same hardware as you will get within Windows.
People are not wrong to be questioning if Apple still considers themselves a "Software" company as it seems that they're intent in going the way Microsoft did with Vista. It isn't uncommon to hear people complain that 4gb of RAM isn't enough for a Mac computer these days, and that is troubling, when Windows and Un*x systems are able to operate quite smoothly on 4, and in some cases even 2.
as for Ubuntu, I don't know the posters motivation, but Ubuntu is seen by linux professionals as a very bloated and one of the worst performing linux variants. The fact that it has better resource handling than OSx should be troubling to all of us who enjoy using OSx since the constant push for higher priced and more resource hardware should be the opposite of where we want to go.