Assume you watch a movie on day 0, mail it on day 1, it gets processed on day 2, and you get a new movie on day 3. Ignore weekends. That is 1 DVD for every 3 days.
Your downspeed bandwidth is for a single-layer DVD is
4.7 x 10^9 bytes / 3 days = 4700000000*8/3/24/3600 bits/second = 145062 bit/s = 145 kbit/s
That, of course, sucks compared to my 1.5 Mbit/s DSL connection. On the other hand, it is three times better than a 56 kbit/s modem.
* * *
A more favorable look: I have a three-disc subscription. 435 kbit/s. Assume all DVDs I get are double-layer. 787 kbit/s. Let's also count the upstream return bandwidth. 1574 kbit/s. Not too shaby.
Conclusion: A significant percentage of the bandwidth to my house comes through the US Postal Service.