Things on the external monitor are larger because the pixels on the screen are physically larger.
Most external desktop monitors have a pixel density of around 100 Pixels-Per-Inch (PPI). This means that one inch has 100 pixels going across it. This is pretty small, but many things go much smaller.
Your Galago has a 14.1 inch display running at 1920x1080. This translates to about 156 Pixels per inch. In order to cram more pixels into the same physical space (one inch), the pixels have to be much smaller -- around 50% in this case.
This is what makes everything smaller on the Galago and larger on the external monitor.
The problem lies with the differences between physical screen size and logical screen size. The physical size of the display is pretty simple to visualize; it's just the distance from one corner to the other. The Logical size is the actual screen resolution. So your Galago and a Bonobo Extreme both have the same logical screen size (since they both have the same resolution), despite having wildly different physical sizes.
It's similar to scaling up an image in Gimp, or zooming in in Firefox. The physical size of the image gets larger, while the logical size (the pixel dimensions of the image) stay the same.
Future versions of Ubuntu that use Mir will allow for resolution independence, where the size of objects on the screen aren't determined by the pixel density. They use special Grid Units (GU) to design an application. Then all of the Grid Units get turned into physical pixels by Mir. So this means that if a Grid Unit is an inch wide, and the Unity Launcher is one Grid Unit wide, then it will be one inch wide on both a high-density display (like your Galago's built-in display) and one inch wide on Low-Density Displays (like the external monitor).
(By the way, thanks for ordering a Galago! I work for System76, and it's always fun to see our stuff in the wild!)