Also, Pokemon with excessive grinding? Pokemon has to be one of RPG series with the least aumont grinding. You pretty much can beat every game with next to no grinding.
Play the the first pokemon (red and blue), pick Charmander and tell me you don't have to grind to beat Brock (the first pokemon trainer), which you reach in less than one hour playing, probably 15 minutes if you don't grind.
Even if you pick Bulbasaur or Squirtle you will have to grind to get the first "elemental" attack from those two (actually for Bulbasaur you need the second elemental attack since the Leech Seed doesn't do much damage at all) so you need to have reached level 13 with Bulbasaurand and level 8 for Squirtle to beat Brock.
Pokemon Yellow is like that too since you start with a Pikachu which gets destroyed by rock types.
For example Pokemon Silver, Gold and Crystal also had you grinding a lot.
And so on, I haven't played the most recent pokemon games and I don't think I will because I don't own a 3DS, but most of the older gens were grindy as hell.
Also Pokemon games do get boring because they are always the same, catch pokemon, battle battle battle to level the pokemon, talk to shallow characters and beat bad shallow characters, rinse and repeat until you become the champion pokemon trainer, this goes for hours and hours and hours. There is not much more to do while you play the game (although they started making pokemon contests and stuff like that to give some variety, but you still pretty much have to do the first thing, rinse and repeat).
About a Fallout game that spans the whole mainland, it would be feasible but not using Bethesda's Creation Engine. There are many games that have huge overhead maps and they work, after all most of the USA is still wastelands with nothing to see. Fallout and Fallout 2 cover a huge area and they work, games like Mount and Blade span an entire continent containing deserts and snowy areas and lots of towns+cities+castles and it still works too.
About getting bored if it is big, for me it always depends how the game is done. For example Skyrim is a big game and I get bored of it in less than one hour because everything is shallow and boring, in Morrowind I get interested in the world because it is interesting even though the dialogue system sucks in that game. Fallout 4 is big and once again I get bored because it is also shallow, Fallout New Vegas I get interested because once again there are interesting things (stories, struggles, characters, etc) there and i would have loved if they could have made the game as big as they had planed in it.
For me what makes games boring is if they are shallow, either in mechanics (do this and repeat over and over), in writing (weak stories, characters, motivations, etc) or world building (how does these people manage to survive if raiders keep attacking them while they have no food sources, why do raiders keep attacking these people when they have nothing at all for the raiders, why are there still medicine in this hospital that have been looted many times over the past 200 years?), etc.
Fun games you will never want them to finish. You might have the option to finish them but you will want to squeeze all you can out of them before that happens, if they are big then it is good because they are fun and interesting. You also do not have to have a big game and have to access every area and town and cave, you can have a main quest that will take you to several locations and you can forget the rest, good games do not make you go to everywhere.
EDIT: I just thought of something, even using Gamebryo TTW merges both Fallout 3 and Fallout New Vegas + all DLCs for both games into a massive game. It is huge and still it is a very popular Fallout New Vegas modding project. We get many people thanking us for it all the time. Big games can be fun even if one half is a more mindless shooting/action with shallow world and characters if you have another half being more serious, interesting and better constructed.