LyX requires a LaTeX (or LuaTeX/XeTeX) installation, and comes with its lot of pros and cons. Access to massive universe of *TeX packages. LyX seems more widely used.
TeX is not several GB. It was designed for systems running under intense resource constraints, and it is tiny.
Most widely used TeX installations (TeXLive is the big one) bundle lots of the most popularly used packages by default, but that's an option, and they can be installed on demand as needed. Even granting the reasonable point that modern TeX is functionally useless without packages (and other essential accouterments such as PDFTeX rather than Knuth's original code), I stripped down TeXLive to the packages I actually need (which is an easy choice in the installer, not some arcane hack), and it's about 300 MB.