What do you mean by this? When are the stages smaller or larger? I found some stages in the game to be very small and others much larger.
When you play those stages in Player Matches or Ranked or even in PQs, they are more open but in Endless Battle & local multiplayer they cut a section from them and turned that section to the area you fight in; hence being downsized compared to how they are in the other modes. This is because of the dynamic camera used where if they kept the original stage size for the stages then there'd be all kinds of problem with the camera dealing with certain obstacles (such as the Time Chamber's building getting in the way) and one of the players (the one the camera wouldn't be behind) being too far away at points.
You'd understand better if you played the stages on Endless Battle.
In other words, some of the stages available are smaller than they are in the other modes.