Across its massive userbase, Ubuntu is used on numerous computers with numerous hardware capabilities and screen sizes, latter hardware component properly met by Unity, a screen space-optimized interface (featuring integrated-into-the-panel menus, on-demand auto-hide behavior, etc).
On both big and small screen sizes, the Unity launcher allows the user to hassle-free organize, manage and access applications, launcher that presents itself as a both running and non-running (yet added to the launcher) apps manager.
Nevertheless, there are situations when, in order to complete a certain workflow and/or while dealing with daily-used applications, the user utilizes a serious number of apps, action that fills the launcher with used-application icons.
Especially on small screen sizes, the above mentioned approach (with 30, 40, etc, apps) tends to clutter the launcher, thus generating a non-optimal desktop experience.
Yet, via CompizConfig Settings Manger and/or
System Settings-->Appearance, the user is able to decrease Unity launcher's icon size to 32 pixels, action sometimes insufficient.
The official Unity launchpad webpage has received an interesting addition (to probably land in Ubuntu 13.04 in the near future), branch that introduces the handy ability to decrease the Unity launcher's icon size as low as 8 pixels, removing the "old" 32 pixels barrier.
According to Ubuntu's Daniel van Vugt: "This only allows sizes below 32 to be set in CCSM. The Appearance control panel probably still limits you to 32", issue probably to be fixed in the future and/or implemented by third-party developers in third-party tweaking utilities.