I am evaluating ways to reduce the overhead of (H)MACs on small messages. I was primarily wondering if there is a difference between halving the MAC length or keeping the same MAC length but using it to authenticate a message of twice the size.
My initial thought was that the message/MAC ration would be the same though then it occured to me that this is probably not the complete picture as each individual package would be simpler to attack. Therefore I would probably have to discard both packages if one if them fail an integrity check.
Is my last conclusion correct for them to behave identically or am I missing something?