You're completely right. A lot to do was made about how huge GTAV's world is and how GTA:SA could fit into it X amount of times, and some such garbage, but even despite that, you are right, GTA:SA's world does feel larger and there is a reason for that: the road design.
GTAV's road design is pretty logical, and there aren't "lost" points of the map... Every part of the map is pretty easy to get to via driving, pretty quickly. now, this is intelligent game design... it cuts down on the amount of pointless driving from A to B and that was a big complaint about GTA:SA... That a lot of the game time is simply spent by the player driving on these long road trips from one corner to another. But, back to GTAV, here is the general map for most of the regions of GTAV.
I've marked with colored stars a bunch of points that the player usually finds himself beginning in. And then with similarly colored arrows I've marked roads that allow the player to get to the further regions of the map very quickly. The one in particular to point out is the Red star in the heart of downtown Los Santos, and then the red arrows pointing to that single road. That road is a 2 - 4 lane road that leads directly out of Downtown Los Santos directly into blaine county, a "remote" part of the world map. If you follow the road, you notice that of course, it jogs to the left when you are in Blaine County and that might take a while to drive around... But, I used the anchor arrow to point to a very obvious off-road portion... you can generally maintain a high speed, and go off-road where that anchor arrow is, and that takes you directly into the Grand Senora Desert where Trevor's air port is, and right into Sandy Shores, where a lot of missions begin for Michael and Trevor. This road alone cuts down what could be a very long drive (say, 5-10 minutes) into maybe 90 seconds (give or take, I could be off here I haven't played in ~6 months).
There are a few other stars that I've highlighted and paths that lead out. The highway design is also very logical so you can get from most parts of Los Santos to other parts of Los Santos and Blaine County in 2 - 3 minutes instead of maybe 20+ minutes. The availability of on ramps and large highways leading in and out and around the map make it very easy to quickly travel many in-game miles.
Now, make no mistake, this is intelligent game design and map layout. It's logical and it would be annoying in today's day and age to have to deal with an illogical road layout. Many of the roads in GTAIV were illogical and ultimately frustrating... When you were in Dukes for those handful of missions after your taxi company is burned down, that area is really isolated, with only 2 highways to get to it and a lot of extra driving... It's annoying (the poor individual road design in GTAIV discouraging high speeds made it worse as well).
But, compare GTAV's larger map with quick road design to GTA:SA's technically smaller map but with a much more trecherous road layout. If you are in Los Santos and you need to get to San Fierro, this is a long drive. Even the most direct route, which involves the freeway that runs the exterior of the map, takes a long time to get to. If you are on Grove Street at CJ's house, you have to travel EAst on the map to get West, and that highway takes you through the industrial district of Los SAntos, all the way around the Airport, through the beaches, near downtown, before that small bridge that takes you into the San Fiero part of the map, and then around that long highway. You could try and cut through the wilderness, but there are a couple of spots that are long climbs up hills that are designed to slow you down and keep you from traveling quickly.
I've highlighted the routes and marked a couple of those spots:
Also, with GTA:SA, the highways were not easy to get onto when you're in downtown Los Santos. If you're in the middle of the city, at ground level, the highways above aren't easily accessible even though you are directly below them. you often times have to drive well out of the way to come back and get on the highway.
now, clearly, this road design is worse for game design, but it has a positive effect... It makes the world of GTA:SA feel that much larger. The mission design does this as well, but especially the road layout. I'll get to that.