The following table that summarizes 1-Series through 7-Series size evolution.
The interesting thing is that the new 2008 1-Series is bigger than the first 3-Series of 1975. Longer, wider, taller, heavier, and way more powerful.
At the same time today's 3-series is larger than the first 5-series of 1972 (except that is a bit shorter).
And finally, today's 5-series is larger than the first 7-series of 1977 (except that is 1 inch shorter).
All series have grown, replacing the next series. We've seen many threads attacking how the 3-series is now heavier, bigger, etc. Well, the 3-series is the not the entry level anymore, it's the new "5-series". And if you research, this is common throughout all brands (look up for instance the first Ford Explorer specs and you'll see that the new Ford Escape is almost the same size as it, and the new Explorer is now a big almost full size SUV).