I'm new to Ubuntu and i'm amazed that open source alternatives to my Windows apps like GIMP, Amarok, Libreoffice, Cheese etc are so much smaller than their Windows compatriots (MS Office, Photoshop, Youcam etc). Ubuntu has to be the best evolution ever! And the best after all these is that it's completely free
You mean smaller on disk? Probably thanks to the extensive use of dynamically linked libraries, that are shared between multiple applications. Of course Windows also supports that, but DLLs aren't easily versioned, and the risk of "DLL hell" doesn't encourage sharing.
There's a number of possible reasons which cannot be valid in general though. The bigger sized software might have more features. Or more cruft. Or both. Or the open source software is just so much more cleverly engineered than its counterpart. Some or all of these may be the reasons in any given case. Or not.
One general reason would be the benefits of centralized software distribution and dependency resolution. In most open source based operating system environments, when a software is installed which depends on other software packages (as is most often the case, e.g. an executable depends on libraries etc.), those dependencies are installed as well - if they haven't been already. So they are installed once, max.
Without centralized software distribution and dependency resolution (e.g. in Windows) such dependencies may often be packaged with the software that needs them. So dependency X may be packaged with software A as well as with B (and C and D and so on) -> same dependencies installed multiple times (or even statically linked into the executable) - just to be on the safe side.
So that might account for some of the differences in size. Surely not all though.