Scaling large file downloads?

This is not a bad architecture (squid is a popular reverse proxy), however if you expect exponential growth a Content Delivery Network could be a better solution - you only pay for content that you require, bandwidth scales instantly (you don't have to scale out more servers) and streaming transfer is geolocated to servers as close to the client as possible, ensuring they get maximum transfer speeds. However I've never tried this with files of 1GB, and the cost may be prohibitive.

Torrent technology can be considered as a p2p CDN in this case, and as such some of these providers may be suitable as torrent seeds for your content, reducing your total bandwidth costs and (possibly) increasing speed, although that's dependent on your leechers.

If you aren't currently doing this, it may be worth investigating delivering the files optionally over bittorrent to push some of the load off of your servers and onto the P2P network.