Not too sure if its updates are in Mbs. Overall directory path has a little over 10MB for the entire program....Pretty impressive. Also even if it downloads large files I am not too worried as I have plenty of bandwidth with little chance of choking.
That being said, call me paranoid but having used Symantec, Avast and Kaspersky, I am a little unused to the calm that I have had with MSE. Also apart from the few initial reviews which are positive, there is not too much coverage of this.
CPU utilization is what concerns me more. Symantec has released a gaming version of Norton recently. Apparently the overhead is minimal but a am yet to try this out.