Iconfactory’s Craig Hockenberry on using vector images for rendering bigger icons:
Unless you’re dealing with simple line art, effects such as gradients, shadows, and highlights result in enormous files. As an example, compare this 512×512 pixel PNG file of the CandyBar icon with a PDF file containing the same image. The PNG file is about 100 KB while its PDF counterpart is a whopping 3 MB. Consider a five icon toolbar with PNG files versus a toolbar with PDF files — 500 KB versus 15 MB. Your ISP will love you and your PDF icons!