so far bitcoin has been out for around 30 months and has only just reached about 400mb, 500 if you count indexes. thats 2.5 years.
yes, but bitcoin is also still waiting for it's major break through and doesn't really have many users and shops yet.
if the first 2.5 years made 400mb, i bet the *next* 2.5 years would easily make an extra 4000mb (if not compressed or pruned).