Linux isn't lightweight by default—here's what actually makes it work on old hardware — A popular project is to install Linux on an old PC. While it's useful for keeping an old machine from the recycler, Linux isn't magic. It's a result of how Linux is designed.
One reason that Linux has such a reputation for being lightweight is that distro developers try to make it so. While there is only one version of Windows, there are many Linux "distributions," or collections of software. A distribution includes the Linux kernel, or the "heart" of the operating system, utility programs, a desktop environment, and user programs. The latter would include a web browser, office software, and simple games, like Windows used to come with.
Linux isn't automatically lighter than Windows. Ubuntu, one of the most popular distributions, particularly for people new to Linux, has a minimum of 4GB of RAM, the same as Windows 11.
Unlike on Windows, the desktop isn't coupled to the main OS. For many years, the main graphics system was the X Window System, or X11, after the latest version published by The X Consortium in 1987.Wayland is a newer system that many popular distros and desktops are switching to.
A desktop itself is made out of smaller components, such as window managers, which draw the buttons and window borders, as well as utility programs like the file manager that lets you graphically browse the file system.
The desktop is just another program and can be switched out easily. The ability for distro developers to change the desktop environment makes it possible to create "lightweight" distros and desktop environments.
The most popular desktops on standard Linux distros are GNOME and KDE Plasma. The developers will choose which one they want. Apart from aesthetics, GNOME, the default desktop of Ubuntu, has a user interface design roughly similar to macOS, so people who use Macs will find it easy to adjust to GNOME. KDE Plasma's design looks a lot like Windows, so Windows users, still the vast majority of desktop users, will similarly find it easy to use KDE Plasma.
There are other smaller desktops, such as Xfce (my favorite), LXQt, and others. Good examples of distros based on them include Kubuntu and Lubuntu. You don't have to install a whole new distro just to use a different desktop. All you have to do is install it on your existing Linux system. You'll be able to switch through the login menu.
Linux has its reputation for lightness due to the fact that it can run different programs. Desktop Linux distros will ship with similar kinds of software to Windows, such as text editors, web browsers, and productivity programs. They may be similar to what you'd find on a Windows system, but the two systems are different.
Linux inherited the Unix philosophy of small programs connected by "pipelines" that connect the flow of output from one program to the next. That's not feasible in graphical software, but I think it's had an influence on the design of Linux programs as an ideal. It would be difficult to describe popular programs like LibreOffice or Firefox as "lightweight," or the 3D games that are becoming more popular on Linux thanks to the Steam Deck.
I suspect if Linux became popular as a desktop operating system, people would likely just complain about "bloated" programs there. Joel Spolsky highlights a problem with a lot of "lightweight" programs: they have a limited range of functions so that anyone who needs something outside that narrow band can't use them. A "bloated" program like Excel has a large user base because someone's workflow often hinges on an obscure function, so the "bloatware" might be easier to use for more people in practice.
The modern web also needs compatible browsers, so a lot of lighter-weight browsers like Dillo are out. Lynx is still good as a text-based browser, especially if you need to make sure that web pages are accessible to people who are blind or who have low vision.
As great as Linux is, you might run into issues with hardware support, though this seems to be less common over time. Linux, as with Windows, needs device drivers to make hardware work. Windows has a much larger share than Linux on the desktop, and hardware makers target that market. Most Linux device drivers are written by amateurs.
Hardware functionality is also often a trade secret, so Linux programmers often have to reverse-engineer devices to write drivers. This can make hardware support hit-or-miss. A lot of Linux users have had the frustrating experience of having only part of a device's features work on Linux or not at all when it fully works on Windows. Newer devices where someone hasn't yet written a driver seem to suffer this, while it can also happen to older or obscure devices.
Laptops that have weird, non-standard motherboard designs can be susceptible, and Wi-Fi was a common pain point on many systems in the 2000s.