It's basically opening a channel. So the amount you put in determines the size of the channel.

I've used Breez but not Phoenix so I'm just basing this on what you've said, but it sounds like with Phoenix the minimum channel size is set high, probably to ensure low friction during use.

On Breez you can send much smaller amounts to open a smaller channel. When you want to send a larger amount it just takes a certain amount off the sats you send in to pay for the bigger channel.

The cost is the same because the amount you can send depends on how many sats you open the channel with, but Breez lets you start small and pay for a bigger channel if you need it, whereas it sounds like Phoenix wants you to pay for a larger channel straight up.

Breez is easier onboarding for a mobile wallet imo.