Draw distance, that is all.
SA felt bigger because you could'nt even see a mile away
Also true, San Andreas Draw Distance was LESS than Vice City and GTA3!! On average, Vice City had roughly 1600 FarClip, Highest at 2000 FarClip, Lowest at 150 FarClip (Fog). San Andreas had an average of 400 FarClip, Highest at 800 FarClip, Lowest at 100 FarClip.
Grand Theft Auto V? Well, let's take a look, far_clip 5000.00. Lowest being 4500.00 for Foggy, the game uses depth of field and blurs and other effects to create the distance hazing and blurs in the game.
edit: So in view, Draw Distance alone, GTAV is slightly greater than 6x the MAXIMUM San Andreas Draw Distance, so for those PC users, go and edit the timecyc to have those farclip values and see how it will make the "massive" San Andreas look.