I just took a look at server and we download new defs every 240 minutes and the clients checking twice a day. The size of the definition files that I am seeing is only about 500KB. But depending on how many types of clients you have you may see more than one at the same time. for example, IA64, x64, and x86. The definitions are different based on the arcitection of the OS.
If you configure a live update policy to have the computers check for live updates wheather they are in the office or not at a specific interval, the definition files will be very small. It is the product updates that make Live update files large.
At least this has been my observation.