.. I think it's because SA focusses on a bigger area than GTA V. It's smaller, but it gives you the feeling of being in a whole state with diverese countryside and three cities. We don't have to talk about the lame draw distance in the old game, we all know that.
GTA V focusses on a very small area but it has more realistic measures, SA was more "toy"-like. I think Chiliad alone would cover more than two SA cities. GTA V's map has more realistic terms of scale, is bigger than old SA but represents just a "tiny" area of the state.
The only thing I dislike is that Trevor's dialogues in the SP story try telling you Sandy Shores is "an hour from Los Santos" or... he told Franklin that Paleto Bay is "four hours from Los Santos".. I had to laugh at these sentences, seriously. GTA V may be big, but - like I said - it focusses on a smaller area and in my opinion you never feel "hours away" or "lost" from Los Santos. I always feel like I'm in a LS-neighbourhood.
I love both maps, but I prefer GTA V's map.