And from the creator himself:

The bandwidth might not be as prohibitive as you think. A typical transaction
would be about 400 bytes (ECC is nicely compact). Each transaction has to be
broadcast twice, so lets say 1KB per transaction. Visa processed 37 billion
transactions in FY2008, or an average of 100 million transactions per day.
That many transactions would take 100GB of bandwidth, or the size of 12 DVD or
2 HD quality movies, or about $18 worth of bandwidth at current prices.

If the network were to get that big, it would take several years, and by then,
sending 2 HD movies over the Internet would probably not seem like a big deal.

Satoshi Nakamoto