(A) The basic question posted by the TS is whether an ISO 400 film will produce "bigger" grains than ISO 200 or 100. Related, but not specifically asked, is the issue of the effects of scanning.
I do not scan my negatives and therefore I am unable to give a coherent answer to the effects of scanning.
While I am going to talk specfically about B&W films, I believe that msot of what I will say should apply to color films too.
But yes! In principle, higher ISO should result in bigger grains. The reason is the size of the silver halides used to "gather" light. A higher ISO film will have bigger grains to start with in order to be more efficient in gathering light. But with new technology, such as the Ilford Delta series, Kodak Tmax series and the Fuji Neopan series, the ability to gather light was made more efficient by the shapes of the silver grains, hence you may find that a higher ISO "T-grain" film may have grains comparable to a lower ISO of the "older" panchromatic films like Kodak TriX, Kodak Plus-X, Ilford FP4, and Ilford HP5.
But this is not the end of the story.
The final "size" of the grains, within the same ISO in the same film, can also be affected by other things. Some people had suggested that different developers may alter the size of the grains. But given the fact that the size of the grains was determined at manufacturing, the difference is not a lot. And indeed, Bruce Barnbaum, one of the acknowledged teachers in B&W printing, believes that the effect of different developers on graininess is at best minimal. Some have felt that staining developers, such as pyro developers, could reduce graininess by "filling" up the space between the silver halides. But again, this is arguable.
What is more important in altering the size of the grains is the LENGTH of development. And also agitation.
I hope that answer the question, with some further clarifications.
(B) Now to correct some rubbish made in some posts.
(1) Films are not imbued with intelligence. Films respond to light, something purely physical, and chemical when you develop the films.
Films do not know whether you are making photographs outdoors or indooors. Films only respond to light. Period.
The issue is not outdoors or indoors.
The issue is a question of adequate exposure. Underexposure, with attempts to "correct" the underexposure by lengthening the development time, such as in "push-processing", will result in more grain. One can photograph in "outdoors" and result in underexposure because the photographer do not know how to expose the negative properly. One can also photograph "indoors" and get correct exposures.
(2) ISO 200 is NOT "good for outdoors but not under low lighting". ISO ANYTHING, in principle, is not good for any underexposure, unless you are looking for certain effects.
(3) ISO 100 is NOT "only suitable for daylight shots". (Yes, I know that whoever said this later contradicted himself in the very next post by saying "in fact ISO 100 films are very good for day and night shots, but a tripod is required"). ISO 100 can be used for any lighting shots, including indoors shots, and night shots, provided you know how to get sufficient exposure. Whether a tripod is required or not is a secondary issue. Some night shots such as hawker centres are very bright, and given good techniques and very fast lenses, using a rangefinder, can even be taken handheld.
(4) ISO 1600 is NOT "very seldom used". On the contrary, ISO 1600, and even higher, is very commonly used in jazz clubs and stage photography. In fact even better, use a 3200 film and rate it at about 2000-2500.
Now I hope these clear up the nonsensical inaccurate misleading information spewed up by half-baked n********ps .
(C) Finally, I use a lot of ISO 400 films. As I mentioned, I do not scan films. But I print them and then scan in the prints. Take a look in the portrait subforum. I have two images there.
One is a nude abstract taken with TriX 400. How is the grain?
The other is a Child and her Candle taken with Fuji Neopan 1600, hand-held using the candle as a light source. Obviously more grainy. But what is the effect?