I have no idea, he didn't say it on his vid. Does the size of the transaction mean how much money he's sending? I thought the whole idea of Bitcoin is that it doesn't matter if you send 1 cent of 1 billion dollars, the fees should be the same.
This idea can still be correct. What increases the transaction size is the amount of inputs and outputs of the said transaction, and not how many Bitcoins have been sent. Usually, a transaction with 1 input and 1 output is equivalent to 250 bytes, independent if this input is worth 0.005BTC or 50BTC.