michaelleung: What? Only 110 GB in total?
That's about 700 MB per game, which seems about right.
ToCA 3 is 5.3GB, Colin McRae is 2.9GB, Tex Murphy: Overseer is 2.7GB, The Feeble files is 2.3GB... those are the biggest games on the site I think (i believe they're counting ToCA and CMR, as they seem to be available for download still, though not for purchase), and the rest are 1.5GB and below, Martian Memorandum + Mean Streets is 27MB, Teenagent is 8MB, Lure of the Temptress is 12MB, Duke Nukem 3D is 27MB Beneath a Steel Sky is 71MB, and those are among the smallest ones (I'd bet there are a couple others smaller then BaSS, but I only included games I've bought myself).
Bottom line: though some games are over 1.5 GB, the majority of them are below 700MB, and that is including compilations of several games (though GOG probably lists them as a single entry, making it a moot point), and for DOS-era games that is not surprising.