even though base64 increases raw size by about a third, this is mitigated by gzip or deflate encoding by the webserver. The actual transmitted size is only about 5% bigger
dd if=/dev/urandom bs=1024 count=64 | base64 | gzip | wc -c
64+0 records in
64+0 records out
65536 bytes (66 kB) copied, 0.0127386 s, 5.1 MB/s
67302
This particular run comes out to ~2.7% overhead, and in fact it's very repeatable. The half dozen runs I did were within 20 bytes of each other. $ wget http://en.wikipedia.org/wiki/ASCII
$ (cat ASCII;dd if=/dev/urandom bs=8000 count=1 | base64) | gzip | wc -c
50578
$ (cat ASCII;dd if=/dev/urandom bs=9000 count=1 | base64) | gzip | wc -c
51621
Around 4% overhead. Either way, it's negligible.