The dimensions in which a roll can be big or small are:
- Absraction - the more of a scene gets resolved in one roll, the bigger it is.
- Player Control - the more the players can affect the roll, the bigger it is - especially if players can affect it at the time of the roll.
- Side Effects - bigger rolls often have consequences that extend beyond a strict determination of success or failure.
The smallest roll is a coin flip for a single action. Heads or tails, success or failure. Neither players nor GMs can affect the probability of the outcome, the 'roll' itself only affects one action, and it doesn't 'mean' anything beyond the very minimum required by a roll in an RPG - does the action succeed?
Another game with large rolls is the new Warhammer Fantasy Roleplay. All the bits and non-standard dice govern not only success and failure, but your character's approach to the situation and positive and negative side effects. The result is fairly baroque, but intuitive enough at the table to do complexity well. WFRP3E's rolls are still as common as a smaller roll - less abstract - but are still large in their level of implication.
Other ways of enlarging rolls (without using funky types of dice) are degrees of success (Alternity, Dark Heresy); complex, multi-roll challenges (D&D4E, both Spycraft editions); and Cool Points or Effort Points or the like (Shadowrun, 7th Sea, even HERO System, with its rules for 'pushing').
Conclusions? I don't really have many. It's just worth thinking about when designing a game. These are gross generalizations, but more abstract rolls tend to favour games less about combat and more about story. Rolls with more player control tend to favour characters that succeed through style or inner strength, rather than strict competence. Side effects can add levels of mechanical or tactical depth, as well as mechanical representations of more story-based consequences, but side effects can also easily be handled inelegantly.