This is because the fee is calculated based on the amount to be transferred, not the data size as with on-chain, so until the transaction is attempted there’s no way to guarantee what the total amount will be. Larger amounts usually require a higher fee because there are fewer channels available to send them through. Usually you end up having to perform multiple smaller transactions to get to a point where you are down to zero (or close to it)
in your wallet.
Could hitting send max be that amount you want transferred though?
For instance let's say I know that I want to move 12345 sats from one lightning wallet to another. I hit send max, it tells me I can send 12300 sats max to another wallet and that the rest of the sats I have will cover the fee. I then go to the other wallet, generate an invoice for 12300, and then conduct the transaction. Result is all sats go over to wallet or are used for fees. Leaving me with 0 sats in old wallet.
Is this not feasible?
Probably not, because your wallet wouldn’t be able to calculate it exactly due to all the variables involved. Usually they just ask you to reserve 3%, so if it’s 10000, you should attempt to send 9700, but if that doesn’t work, you can try sending 9000 and break up the remaining 1000. You will almost definitely pay lower fees if you don’t try to send it all at once.
