They are really small because the technology used to create them is based on mathematics, so it only stores maths-variables and can achieve very high quality. The downside is that the computer needs to spend format out there. Being vector graphics, SVG is very light weight. This is because it usually only stores coordinates and parameters for the maths involved with vector graphics. It is maintained by the save Photoshop style layered .tiff. These are different from regular layered .tiff, as Photoshop stores *.psd data inside the .tiff. This means things like layerstyles and blending modes can be stored