Phoenix takes 15k (usually aroun 3k at lower fees) to open and/or enlarge channel.
So if say you have 2m sats to put in and you send in 1 m
Phoenix will open a channel for you with maybe around 1.2 m capacity, meaning you can receive 200k more.
Now if you want to transfer in the other 1m you will not have enough inbound capacity so Phoenix will need to do another on chain tx to enlarge your channel which is another 15k at the moment.
But now you'll have a channel with around 2.4m capacity which is yours, forever so if you sent the sats back out you will have more inbound liquidity to receive more.
The same thing applies if you run your own lightning node and manage your own channels. Custodial solutions take care of all these complications behind the scene.