I suggest that you should first contemplate what the term, "lightweight," means to you.

By default, distros seem to be designed (quite sensibly, I might add...) to "install on everything, no questions asked." So, they tend to load driver modules for everything under the sun, then to execute complicated startup-scripts to decide which modules to load. (I vividly remember noticing that a Red Hat distro contained a driver for an obscure DEC (as in, Digital Equipment Corporation, R.I.P...) token-ring interface card.)

You can often pare things down considerably by observing which packages you actually need and unloading the ones that you determine that you don't. This applies not only to kernel modules, which are used to "personalize" the kernel at startup (and for other purposes), but also, and perhaps especially, for user-land.

Unfortunately, "user-land can get annoying," because you might find that you do need a lot more stuff than you'd prefer to. For instance, you might say, "I either want KDE or I want Gnome or I want Evolution or whatever..." only to find that there's one program that you can't live without which was written for one or the other.

So... consider carefully what you mean by, "lightweight," and start by jettisoning packages that you know that you don't need. (And, if the system tells you that you do, in fact, "need it," believe it.