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This is true for normal lightning.
But splicing makes it different, I think.
In phoenix I always only have one channel and it is between me and phoenix. Any onchain transaction i make affects the size of this channel.
If I make a transaction sending from my phoenix wallet to my on chain, it has to change the size of the channel, even if it is only a small percent that you send out.
If it doesn't change the channel size, where do the sats come from?
I have done exactly this multiple times, and it reduced my channel size by the amount I sent out.
That's the thing, you are using internal swap. Use external swap (sending a normal LN payment to the swap) and you will see that will not reduce the size. Is also cheaper using an external swap.
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Yes. I think the OP used the internal swap and that was why he was confused.
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