A giant was a creature from Human folklore described as being Human in appearance with a prodigious height.
In 2268, Leonard McCoy described the Arretans metaphorically as being "giants, and we're insects beside them" (i.e. their technology was much more advanced than that of the Federation. (TOS: "Return to Tomorrow")
In his conversation with Fleet Admiral Kirsten Clancy about the planned Romulan attack on Coppelius in 2399, retired Admiral Jean-Luc Picard said that the windmills had become giants. (PIC: "Broken Pieces")
Although not specified on screen, Picard's metaphor was presumably a reference to the 17th century novel Don Quijote by Miguel de Cervantes.