This is a feature in the browser version of Agar.io which allows the player to zoom out and zoom in. Zooming out allows players to get a larger view of the map, with many more cells in view. Zooming in allows players to get a more condensed view of the map, with fewer cells in view. To change the level of zoom, use the mouse wheel. The level of zoom is relative to the number of cells of the player and their size.
In the spectator mode it is possible to unlock he camera so that it is no longer tied to a player. In this case the zoom level has a fixed maximum. Therefore, if you spectate a specific player and then unlock the camera, you may see that the field of view is actually becoming smaller.
There exist custom extensions that allow to zoom out as much as the player wants to. However, the number of visible objects does not increase.