Are hardlinks responsible for the bitcoin data directory appearing larger than expected? I was running bitcoind from my Linux box. I stopped the command when ~/.bitcoin size was about 5.8GB.

Today I started the daemon again. ~/.bitcoin size is now 7.2GB. Here is says that the blockchain weight about 6GB: http://blockchain.info/charts/blocks-size.

Is that webpage wrong? Maybe bitcoind is downloading everything again and mixing both downloaded block, thus increasing the size to more than the stats at blockchain says.

A call to getblockcount returns: 225531

EDIT: I've found the total blocks count here: http://blockexplorer.com/q/getblockcount (228972). I guess blockchain.info is not updated.
The cumulative filesize of the bitcoind/bitcoin-qt directory has been a source of confusion since the introduction of v0.8 because it hardlinks multiple files within the directory. Briefly, two files point to the same data on the disk. So if you sum the size of the files, you don't get the total amount of data actually on the disk. In this manner, you can put 10GB of files on a 1GB USB, for example. 

Can I resume a blockchain download?

Yes, absolutely

Is blockchain.info is updated?

Currently, yes.

Is the size of the bitcoin data directory easy to understand

Somewhat, you need to understand hardlinks. Your OS might double count some files, leading to confusion.