littlemamas,
the trick is in 'differential' updating.
Lets say that today we have these definitions in VPS file:
sig A, sig B, sig C.
The next update would have:
sig A, sig D, sig B, sig C.
Instead of sending the whole update, we send only info:
read old file, copy 1 sig, insert sig D, copy 2 sigs, save new file.
The same applies to program files.