Rabbit is a symmetric cypher, and Mega's mention of 2048 is in respect to RSA, an asymmetric cypher. Even if Mega were using RSA to encrypt the entire file (as opposed to an encryption key which is then used to encrypt the file, which I personally find more likely, given the performance constraints and various other limitations of RSA, but maybe they really are doing that: I'm not an expert ;P), 2048-bit RSA is said by RSA themselves to be equivalent to a symmetric cypher with a 112-bit key.

> As of 2003 RSA Security claims that 1024-bit RSA keys are equivalent in strength to 80-bit symmetric keys, 2048-bit RSA keys to 112-bit symmetric keys and 3072-bit RSA keys to 128-bit symmetric keys.

-- http://en.wikipedia.org/wiki/Key_size

(Although, this is almost certainly ignoring mathematical attacks, as those would make things highly variable: one could imagine a really lame cypher that doesn't effectively utilize the key, or which leaks the key into the cyphertext. The choice "Rabbit" is thereby important, and Wikipedia mentions that if you want to break a large number of keys, such as for a large number of related files/accounts on a file-storage service, and just need to break any one, a 128-bit key is only about as effective as a 96-bit one.)

> Rabbit claims 128-bit security against attackers whose target is one specific key. If, however, the attacker targets a large number of keys at once and does not really care which one he breaks, then the small IV size results in a reduced security level of 96 bit. This is due to generic TMD trade-off attacks.

-- http://en.wikipedia.org/wiki/Rabbit_(cipher)