Painting - big roller vs. small roller

What is the differnce (other than size) between the big paint roller and the small ones. I dont have one on me but Im guessing the small ones are about 7 inches, and the bigger ones are 9-10 inches and a bigger diameter.

Im assuming - if you paint - you know what I mean.

So, why choose one over the other? Is it based on the paint, size of the room, anything? Is there any difference? Is one better than the other?

I assume you are talking about a standard 9" roller [not a big 18" or one of the various small rollers] Roller covers [often called sleeves] come in different nap sizes [along with being made of different materials]

1/4" nap is used for very slick surfaces - generally when rolling enamel

3/8" also for slick surfaces, holds a little more paint but leaves a little more texture to the paint.

1/2" usually for walls again the bigger the nap the more paint it holds

3/4" for a slightly rougher surface [like ceilings] or when you need the roller to hold more material.

1" - 1.5" for very rough surfaces - holds a lot of paint.

Roller covers are chosen based on both the type of material being applied and the surface the paint is going on.

The small ones are 4", used mostly for small work like cabinet doors.

For painting a room - use a 9" sleeve per Mark's directions.

We usually use the 9" rollers for most of our work.
We do use the 7" for cabinets, doors, shelves, etc, never for walls or ceilings.
The only 7" covers (sleeves) I keep in stock are 1/4" mohair.