Nobody seems to be willing to burn their fingers on this one.
And to be honest, I don't know the answer either. I'm not running a large website myself, after all.
But I got a few remarks.
1) Large content-providers do not have their web-hosting machines located in one single location. Companies like Google, Facebook and others have data-centers on many continents. I also believe Google has deals with large ISPs to deploy caching-servers in the ISPs networks, to improve performance.
2) Even when you don't have the money to build multiple data-centers, you can still do all kinds of caching tricks. Companies like Akamai and CloudFlare have so-called Content Delivery Networks (CDN). Your webpages will be distributed to the CDN-servers around the globe. And users worldwide will have better performance (and reliability).
So it seems people do think that having a webserver on one continent, and customers on other continents, is not an optimal solution. In how far it is not optimal, but still acceptable, I don't know.
3) It's not all about the size of the webpages. The amount of round-trip-times is just as important (or more important). A typical webpage is not a single document, but consists of a few (or many) separate documents. HTML page, jpgs, flash, etc. Depending on technology used in browsers, servers and the webpages themselves, a user viewing a single page might need to do multiple transfers. For every transfer, the RTT adds up. Browsers don't do a new TCP connection for each document anymore, but they still need to do request/reply. I'm not an expert on web-issues, hopefully someone else on this forum knows more.
There has been noticable improvement on this issue over the years. I used to frequent a forum in Australia very often (and I'm on europe). That was *really* slow. However, with the improvements in browsers and http, performance has improved from hairpulling-slow to reasonably fast now.