What do you know, you're right. I am impressed; I was under the impression that because of the encryption, Dropbox couldn't reliably determine what pieces changed.
Truecrypt is designed to work on a block-by-block basis or else it would be horribly slow at everything. So dropbox's job isn't harder than any arbitrary binary file with a few KB changing. Easier, really, since nothing changes size and shifts the data after it.
