I use glfw v2.5 and I find two glfw.dll.
One is a \lib\win32\glfw.dll another is a \support\win32dll.
Why different size each other?
I noticed this too. It seems to have something to do with which compiler is used to create the dll. For example, I found mingw gcc and openwatcom both create different dll sizes; these dll's are much larger than the one included with the project. I also found that there's a difference in frame rate depending on which dll is used. I don't know why, but one compiler seems to create a dll that runs at almost half the speed of the other. I can't remember which one does that, though.