So here is the thing , sometimes the Size doesn't even reach 1mb but the amounts in "Total sent" is higher then the blocks where there is a lower amount of BTC sent , so I'm really wondering how is the size calculated ? I'm pretty sure It has nothing to do with how much BTC is sent or received.
To the question "how is it calculated?", the answer is "clearly, quite poorly". This is Blokchain.info, they've made a USP out of this sort of mistake for a while now.
General answer is: inputs. How many inputs are you using for your transaction, the more those mount up, the larger the overall transaction will be on the blockchain.